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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- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
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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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> toolkit check pr
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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)

crates/nu-command/tests/commands/save.rs

@@ -91,8 +91,8 @@ fn save_stderr_and_stdout_to_afame_file() {
         let actual = nu!(
             cwd: dirs.root(),
             r#"
-            let-env FOO = "bar";
-            let-env BAZ = "ZZZ";
+            $env.FOO = "bar";
+            $env.BAZ = "ZZZ";
             do -c {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -r save_test_5/new-file.txt --stderr save_test_5/new-file.txt
             "#,
         );
@@ -113,8 +113,8 @@ fn save_stderr_and_stdout_to_diff_file() {
         nu!(
             cwd: dirs.root(),
             r#"
-            let-env FOO = "bar";
-            let-env BAZ = "ZZZ";
+            $env.FOO = "bar";
+            $env.BAZ = "ZZZ";
             do -c {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -r save_test_6/log.txt --stderr save_test_6/err.txt
             "#,
         );
@@ -206,8 +206,8 @@ fn save_append_works_on_stderr() {
         nu!(
             cwd: dirs.root(),
             r#"
-            let-env FOO = " New";
-            let-env BAZ = " New Err";
+            $env.FOO = " New";
+            $env.BAZ = " New Err";
             do -i {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -a -r save_test_11/log.txt --stderr save_test_11/err.txt"#,
         );
 
@@ -227,8 +227,8 @@ fn save_not_overrides_err_by_default() {
         let actual = nu!(
             cwd: dirs.root(),
             r#"
-            let-env FOO = " New";
-            let-env BAZ = " New Err";
+            $env.FOO = " New";
+            $env.BAZ = " New Err";
             do -i {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -r save_test_12/log.txt --stderr save_test_12/err.txt"#,
         );
 
@@ -250,8 +250,8 @@ fn save_override_works_stderr() {
         nu!(
             cwd: dirs.root(),
             r#"
-            let-env FOO = "New";
-            let-env BAZ = "New Err";
+            $env.FOO = "New";
+            $env.BAZ = "New Err";
             do -i {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -f -r save_test_13/log.txt --stderr save_test_13/err.txt"#,
         );