Canonicalize config dir (#12136)

It turns out that my previous PR,
https://github.com/nushell/nushell/pull/11999, didn't properly
canonicalize `$nu.default-config-dir` in a scenario where
`XDG_CONFIG_HOME` (or the equivalent on each platform) was a symlink. To
remedy that, this PR makes `nu_path::config_dir()` return a
canonicalized path. This probably shouldn't break anything (except maybe
tests relying on the old behavior), since the canonical path will be
equivalent to non-canonical paths.

# User-Facing Changes

A user may get a path with symlinks resolved and `..`s replaced where
they previously didn't. I'm not sure where this would happen, though,
and anyway, the canonical path is probably the "correct" thing to
present to the user. We're using `omnipath` to make the path presentable
to the user on Windows, so there's no danger of someone getting an path
with `\\?` there.

# Tests + Formatting

The tests for config files have been updated to run the binary using the
`Director` so that it has access to the `XDG_CONFIG_HOME`/`HOME`
environment variables to be able to change the config directory.

crates/nu-path/src/helpers.rs

@@ -7,7 +7,7 @@ pub fn home_dir() -> Option<PathBuf> {
 }
 
 pub fn config_dir() -> Option<PathBuf> {
-    dirs_next::config_dir()
+    dirs_next::config_dir().map(|path| canonicalize(&path).unwrap_or(path))
 }
 
 #[cfg(windows)]