Fix hidden env vars not being hidden in closures (#8055)

# Description

This one fixes env not being hidden inside closure, reported in the
conversation under https://github.com/nushell/nushell/issues/6593

https://github.com/nushell/nushell/issues/6593
https://github.com/nushell/nushell/issues/7937 still persist. These
seems a bit more involved and might need hidden env tracking also in the
engine state... I'm not yet sure what's causing it.

Also re-enables some env-related tests and removes unused Value clone.

# User-Facing Changes

Just a bugfix

# 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.

tests/shell/environment/env.rs

@@ -3,6 +3,7 @@ use super::support::Trusted;
 use nu_test_support::fs::Stub::FileWithContent;
 use nu_test_support::nu;
 use nu_test_support::playground::Playground;
+use nu_test_support::{nu_repl_code, pipeline};
 
 use serial_test::serial;
 
@@ -106,17 +107,6 @@ fn load_env_pwd_env_var_fails() {
     assert!(actual.err.contains("automatic_env_var_set_manually"));
 }
 
-// FIXME: for some reason Nu is attempting to execute foo in `let-env FOO = foo`
-#[ignore]
-#[test]
-fn passes_let_env_env_var_to_external_process() {
-    let actual = nu!(cwd: ".", r#"
-        let-env FOO = foo
-        nu --testbin echo_env FOO
-        "#);
-    assert_eq!(actual.out, "foo");
-}
-
 #[test]
 fn passes_with_env_env_var_to_external_process() {
     let actual = nu!(cwd: ".", r#"
@@ -158,3 +148,19 @@ fn passes_env_from_local_cfg_to_external_process() {
         assert_eq!(actual.out, "foo");
     })
 }
+
+#[test]
+fn hides_env_in_block() {
+    let inp = &[
+        "let-env foo = 'foo'",
+        "hide-env foo",
+        "let b = {|| $env.foo }",
+        "do $b",
+    ];
+
+    let actual = nu!(cwd: "tests/shell/environment", pipeline(&inp.join("; ")));
+    let actual_repl = nu!(cwd: "tests/shell/environment", nu_repl_code(inp));
+
+    assert!(actual.err.contains("column_not_found"));
+    assert!(actual_repl.err.contains("column_not_found"));
+}