remove let-env, focus on mutating $env (#9574)

# Description

For years, Nushell has used `let-env` to set a single environment
variable. As our work on scoping continued, we refined what it meant for
a variable to be in scope using `let` but never updated how `let-env`
would work. Instead, `let-env` confusingly created mutations to the
command's copy of `$env`.

So, to help fix the mental model and point people to the right way of
thinking about what changing the environment means, this PR removes
`let-env` to encourage people to think of it as updating the command's
environment variable via mutation.

Before:

```
let-env FOO = "BAR"
```

Now:

```
$env.FOO = "BAR"
```

It's also a good reminder that the environment owned by the command is
in the `$env` variable rather than global like it is in other shells.

# User-Facing Changes

BREAKING CHANGE BREAKING CHANGE

This completely removes `let-env FOO = "BAR"` so that we can focus on
`$env.FOO = "BAR"`.

# Tests + Formatting
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- `cargo fmt --all -- --check` to check standard code formatting (`cargo
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
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# After / Before Submitting
integration scripts to update:
- ✔️
[starship](https://github.com/starship/starship/blob/master/src/init/starship.nu)
- ✔️
[virtualenv](https://github.com/pypa/virtualenv/blob/main/src/virtualenv/activation/nushell/activate.nu)
- ✔️
[atuin](https://github.com/ellie/atuin/blob/main/atuin/src/shell/atuin.nu)
(PR: https://github.com/ellie/atuin/pull/1080)
- ❌
[zoxide](https://github.com/ajeetdsouza/zoxide/blob/main/templates/nushell.txt)
(PR: https://github.com/ajeetdsouza/zoxide/pull/587)
- ✔️
[oh-my-posh](https://github.com/JanDeDobbeleer/oh-my-posh/blob/main/src/shell/scripts/omp.nu)
(pr: https://github.com/JanDeDobbeleer/oh-my-posh/pull/4011)

tests/shell/pipeline/commands/internal.rs

@@ -368,11 +368,11 @@ fn let_doesnt_leak() {
 }
 
 #[test]
-fn let_env_variable() {
+fn mutate_env_variable() {
     let actual = nu!(
         cwd: ".",
         r#"
-            let-env TESTENVVAR = "hello world"
+            $env.TESTENVVAR = "hello world"
             echo $env.TESTENVVAR
         "#
     );
@@ -381,11 +381,11 @@ fn let_env_variable() {
 }
 
 #[test]
-fn let_env_hides_variable() {
+fn mutate_env_hides_variable() {
     let actual = nu!(
         cwd: ".",
         r#"
-            let-env TESTENVVAR = "hello world"
+            $env.TESTENVVAR = "hello world"
             print $env.TESTENVVAR
             hide-env TESTENVVAR
             print $env.TESTENVVAR
@@ -397,11 +397,11 @@ fn let_env_hides_variable() {
 }
 
 #[test]
-fn let_env_hides_variable_in_parent_scope() {
+fn mutate_env_hides_variable_in_parent_scope() {
     let actual = nu!(
         cwd: ".",
         r#"
-            let-env TESTENVVAR = "hello world"
+            $env.TESTENVVAR = "hello world"
             print $env.TESTENVVAR
             do {
                 hide-env TESTENVVAR
@@ -420,7 +420,7 @@ fn unlet_env_variable() {
     let actual = nu!(
         cwd: ".",
         r#"
-            let-env TEST_VAR = "hello world"
+            $env.TEST_VAR = "hello world"
             hide-env TEST_VAR
             echo $env.TEST_VAR
         "#
@@ -446,10 +446,10 @@ fn unlet_variable_in_parent_scope() {
     let actual = nu!(
         cwd: ".",
         r#"
-            let-env DEBUG = "1"
+            $env.DEBUG = "1"
             print $env.DEBUG
             do {
-                let-env DEBUG = "2"
+                $env.DEBUG = "2"
                 print $env.DEBUG
                 hide-env DEBUG
                 print $env.DEBUG
@@ -462,11 +462,11 @@ fn unlet_variable_in_parent_scope() {
 }
 
 #[test]
-fn let_env_doesnt_leak() {
+fn mutate_env_doesnt_leak() {
     let actual = nu!(
         cwd: ".",
         r#"
-        do { let-env xyz = "my message" }; echo $env.xyz
+        do { $env.xyz = "my message" }; echo $env.xyz
         "#
     );
 
@@ -474,11 +474,11 @@ fn let_env_doesnt_leak() {
 }
 
 #[test]
-fn proper_shadow_let_env_aliases() {
+fn proper_shadow_mutate_env_aliases() {
     let actual = nu!(
         cwd: ".",
         r#"
-        let-env DEBUG = "true"; print $env.DEBUG | table; do { let-env DEBUG = "false"; print $env.DEBUG } | table; print $env.DEBUG
+        $env.DEBUG = "true"; print $env.DEBUG | table; do { $env.DEBUG = "false"; print $env.DEBUG } | table; print $env.DEBUG
         "#
     );
     assert_eq!(actual.out, "truefalsetrue");
@@ -527,7 +527,7 @@ fn proper_shadow_load_env_aliases() {
     let actual = nu!(
         cwd: ".",
         r#"
-        let-env DEBUG = "true"; print $env.DEBUG | table; do { echo {DEBUG: "false"} | load-env; print $env.DEBUG } | table; print $env.DEBUG
+        $env.DEBUG = "true"; print $env.DEBUG | table; do { echo {DEBUG: "false"} | load-env; print $env.DEBUG } | table; print $env.DEBUG
         "#
     );
     assert_eq!(actual.out, "truefalsetrue");
@@ -540,7 +540,7 @@ fn load_env_can_hide_var_envs() {
     let actual = nu!(
         cwd: ".",
         r#"
-        let-env DEBUG = "1"
+        $env.DEBUG = "1"
         echo $env.DEBUG
         load-env [[name, value]; [DEBUG null]]
         echo $env.DEBUG
@@ -558,7 +558,7 @@ fn load_env_can_hide_var_envs_in_parent_scope() {
     let actual = nu!(
         cwd: ".",
         r#"
-        let-env DEBUG = "1"
+        $env.DEBUG = "1"
         echo $env.DEBUG
         do {
             load-env [[name, value]; [DEBUG null]]