Remove the 'env' command, as we have the variable (#8185)

# Description

Removes the `env` command, as the `$env` is generally a much better
experience.

# User-Facing Changes

Breaking change: Removes `env`.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

src/tests/test_parser.rs

@@ -234,7 +234,7 @@ fn equals_separates_long_flag() -> TestResult {
 fn let_env_expressions() -> TestResult {
     let env = HashMap::from([("VENV_OLD_PATH", "Foobar"), ("Path", "Quux")]);
     run_test_with_env(
-        r#"let-env Path = if (env | any {|x| $x.name == VENV_OLD_PATH}) { $env.VENV_OLD_PATH } else { $env.Path }; echo $env.Path"#,
+        r#"let-env Path = if ($env | columns | "VENV_OLD_PATH" in $in) { $env.VENV_OLD_PATH } else { $env.Path }; echo $env.Path"#,
         "Foobar",
         &env,
     )