Reorder export-env eval and allow reloading an overlay (#7231)

# Description

This PR is a response to the issues raised in
https://github.com/nushell/nushell/pull/7087. It consists of two
changes:
* `export-env`, when evaluated in `overlay use`, will see the original
environment. Previously, it would see the environment from previous
overlay activation.
* Added a new `--reload` flag that reloads the overlay. Custom
definitions will be kept but the original definitions and environment
will be reloaded.

This enables a pattern when an overlay is supposed to shadow an existing
environment variable, such as `PROMPT_COMMAND`, but `overlay use` would
keep loading the value from the first activation. You can easily test it
by defining a module
```
module prompt {
    export-env {
        let-env PROMPT_COMMAND = (date now | into string)
    }
}
```
Calling `overlay use prompt` for the first time changes the prompt to
the current time, however, subsequent calls of `overlay use` won't
change the time. That's because overlays, once activated, store their
state so they can be hidden and restored at later time. To force-reload
the environment, use the new flag: Calling `overlay use --reload prompt`
repeatedly now updates the prompt with the current time each time.

# User-Facing Changes

* When calling `overlay use`, if the module has an `export-env` block,
the block will see the environment as it is _before_ the overlay is
activated. Previously, it was _after_.
* A new `overlay use --reload` flag.

# 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-parser/src/parse_keywords.rs

@@ -2441,6 +2441,7 @@ pub fn parse_overlay_use(
     };
 
     let has_prefix = call.has_flag("prefix");
+    let do_reload = call.has_flag("reload");
 
     let pipeline = Pipeline::from_vec(vec![Expression {
         expr: Expr::Call(call.clone()),
@@ -2498,7 +2499,7 @@ pub fn parse_overlay_use(
         let module_id = overlay_frame.origin;
 
         if let Some(new_module_id) = working_set.find_module(overlay_name.as_bytes()) {
-            if module_id == new_module_id {
+            if !do_reload && (module_id == new_module_id) {
                 (overlay_name, Module::new(), module_id, false)
             } else {
                 // The origin module of an overlay changed => update it
@@ -2594,13 +2595,17 @@ pub fn parse_overlay_use(
         }
     };
 
-    let (decls_to_lay, aliases_to_lay) = if has_prefix {
-        (
-            origin_module.decls_with_head(final_overlay_name.as_bytes()),
-            origin_module.aliases_with_head(final_overlay_name.as_bytes()),
-        )
+    let (decls_to_lay, aliases_to_lay) = if is_module_updated {
+        if has_prefix {
+            (
+                origin_module.decls_with_head(final_overlay_name.as_bytes()),
+                origin_module.aliases_with_head(final_overlay_name.as_bytes()),
+            )
+        } else {
+            (origin_module.decls(), origin_module.aliases())
+        }
     } else {
-        (origin_module.decls(), origin_module.aliases())
+        (vec![], vec![])
     };
 
     working_set.add_overlay(