fix completions PATH vs Path (#8003)

# Description

This PR attempts to fix the completions issue where, on Windows,
completions wouldn't get generated from items in your path environment
variable. This seemed to be down to `PATH` vs `Path`. So, I tried to add
a new function that we can use anywhere to avoid this problem.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# 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-cli/tests/support/completions_helpers.rs

@@ -43,6 +43,22 @@ pub fn new_engine() -> (PathBuf, String, EngineState, Stack) {
             span: nu_protocol::Span::new(0, dir_str.len()),
         },
     );
+    #[cfg(windows)]
+    stack.add_env_var(
+        "Path".to_string(),
+        Value::String {
+            val: "c:\\some\\path;c:\\some\\other\\path".to_string(),
+            span: nu_protocol::Span::new(0, dir_str.len()),
+        },
+    );
+    #[cfg(not(windows))]
+    stack.add_env_var(
+        "PATH".to_string(),
+        Value::String {
+            val: "/some/path:/some/other/path".to_string(),
+            span: nu_protocol::Span::new(0, dir_str.len()),
+        },
+    );
 
     // Merge environment into the permanent state
     let merge_result = engine_state.merge_env(&mut stack, &dir);