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.
