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/src/completions/command_completions.rs

@@ -37,7 +37,8 @@ impl CommandCompletion {
     ) -> Vec<String> {
         let mut executables = vec![];
 
-        let paths = self.engine_state.get_env_var("PATH");
+        // os agnostic way to get the PATH env var
+        let paths = self.engine_state.get_path_env_var();
 
         if let Some(paths) = paths {
             if let Ok(paths) = paths.as_list() {