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-command/src/core_commands/overlay/use_.rs

@@ -37,6 +37,11 @@ impl Command for OverlayUse {
                 "Prepend module name to the imported commands and aliases",
                 Some('p'),
             )
+            .switch(
+                "reload",
+                "If the overlay already exists, reload its definitions and environment.",
+                Some('r'),
+            )
             .category(Category::Core)
     }
 
@@ -59,7 +64,7 @@ impl Command for OverlayUse {
         let mut name_arg: Spanned<String> = call.req(engine_state, caller_stack, 0)?;
         name_arg.item = trim_quotes_str(&name_arg.item).to_string();
 
-        let origin_module_id = if let Some(overlay_expr) = call.positional_nth(0) {
+        let maybe_origin_module_id = if let Some(overlay_expr) = call.positional_nth(0) {
             if let Expr::Overlay(module_id) = overlay_expr.expr {
                 module_id
             } else {
@@ -114,9 +119,7 @@ impl Command for OverlayUse {
             ));
         };
 
-        caller_stack.add_overlay(overlay_name);
-
-        if let Some(module_id) = origin_module_id {
+        if let Some(module_id) = maybe_origin_module_id {
             // Add environment variables only if:
             // a) adding a new overlay
             // b) refreshing an active overlay (the origin module changed)
@@ -152,6 +155,9 @@ impl Command for OverlayUse {
                     call.redirect_stderr,
                 );
 
+                // The export-env block should see the env vars *before* activating this overlay
+                caller_stack.add_overlay(overlay_name);
+
                 // Merge the block's environment to the current stack
                 redirect_env(engine_state, caller_stack, &callee_stack);
 
@@ -159,7 +165,11 @@ impl Command for OverlayUse {
                     // Remove the file-relative PWD, if the argument is a valid path
                     caller_stack.remove_env_var(engine_state, "FILE_PWD");
                 }
+            } else {
+                caller_stack.add_overlay(overlay_name);
             }
+        } else {
+            caller_stack.add_overlay(overlay_name);
         }
 
         Ok(PipelineData::new(call.head))