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.

crates/nu-protocol/src/engine/stack.rs

@@ -163,7 +163,7 @@ impl Stack {
         Stack {
             vars: captures.clone(),
             env_vars,
-            env_hidden: HashMap::new(),
+            env_hidden: self.env_hidden.clone(),
             active_overlays: self.active_overlays.clone(),
             recursion_count: self.recursion_count.to_owned(),
             profiling_config: self.profiling_config.clone(),
@@ -189,7 +189,7 @@ impl Stack {
         Stack {
             vars,
             env_vars,
-            env_hidden: HashMap::new(),
+            env_hidden: self.env_hidden.clone(),
             active_overlays: self.active_overlays.clone(),
             recursion_count: self.recursion_count.to_owned(),
             profiling_config: self.profiling_config.clone(),
@@ -348,12 +348,12 @@ impl Stack {
         false
     }
 
-    pub fn remove_env_var(&mut self, engine_state: &EngineState, name: &str) -> Option<Value> {
+    pub fn remove_env_var(&mut self, engine_state: &EngineState, name: &str) -> bool {
         for scope in self.env_vars.iter_mut().rev() {
             for active_overlay in self.active_overlays.iter().rev() {
                 if let Some(env_vars) = scope.get_mut(active_overlay) {
-                    if let Some(v) = env_vars.remove(name) {
-                        return Some(v);
+                    if env_vars.remove(name).is_some() {
+                        return true;
                     }
                 }
             }
@@ -361,7 +361,7 @@ impl Stack {
 
         for active_overlay in self.active_overlays.iter().rev() {
             if let Some(env_vars) = engine_state.env_vars.get(active_overlay) {
-                if let Some(val) = env_vars.get(name) {
+                if env_vars.get(name).is_some() {
                     if let Some(env_hidden) = self.env_hidden.get_mut(active_overlay) {
                         env_hidden.insert(name.into());
                     } else {
@@ -369,12 +369,12 @@ impl Stack {
                             .insert(active_overlay.into(), HashSet::from([name.into()]));
                     }
 
-                    return Some(val.clone());
+                    return true;
                 }
             }
         }
 
-        None
+        false
     }
 
     pub fn has_env_overlay(&self, name: &str, engine_state: &EngineState) -> bool {