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
automatically
> toolkit check pr
> ```
-->

# 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-std/std/log.nu

@@ -1,5 +1,5 @@
 export-env {
-    let-env LOG_ANSI = {
+    $env.LOG_ANSI = {
         "CRITICAL": (ansi red_bold),
         "ERROR": (ansi red),
         "WARNING": (ansi yellow),
@@ -7,7 +7,7 @@ export-env {
         "DEBUG": (ansi default_dimmed)
     }
 
-    let-env LOG_LEVEL = {
+    $env.LOG_LEVEL = {
         "CRITICAL": 50,
         "ERROR": 40,
         "WARNING": 30,
@@ -15,7 +15,7 @@ export-env {
         "DEBUG": 10
     }
 
-    let-env LOG_PREFIX = {
+    $env.LOG_PREFIX = {
         "CRITICAL": "CRT",
         "ERROR": "ERR",
         "WARNING": "WRN",
@@ -23,7 +23,7 @@ export-env {
         "DEBUG": "DBG"
     }
 
-    let-env LOG_SHORT_PREFIX = {
+    $env.LOG_SHORT_PREFIX = {
         "CRITICAL": "C",
         "ERROR": "E",
         "WARNING": "W",
@@ -31,7 +31,7 @@ export-env {
         "DEBUG": "D"
     }
 
-    let-env LOG_FORMAT = $"%ANSI_START%%DATE%|%LEVEL%|(ansi u)%MSG%%ANSI_STOP%"
+    $env.LOG_FORMAT = $"%ANSI_START%%DATE%|%LEVEL%|(ansi u)%MSG%%ANSI_STOP%"
 }
 
 def log-types [] {