Ensure cache directory is an absolute path

[bengsparks]

Closes #467. Forces an absolute path by calling Path.absolute.
Tested by repeating the MREs from the aforementioned issue.

The early return of return TemporaryDirectory() a few lines above the diff in the first commit does not affect this,
as TemporaryDirectory will always return an absolute pathname because it uses the same rules as mkdtemp.

Specifically, for Python 3.11 and lower, mkdtemp will always return an absolute path when dir is None, and since Python 3.12, will always return an absolute pathname.

NOTE: It appears that when using env HOME=., a later call to nom-shell after the packages have been built suffers from a similar issue.
error: string './.config/nixpkgs/config.nix' doesn't represent an absolute path.

The call in question

$ /etc/profiles/per-user/ben/bin/nom-shell \
    --argstr local-system aarch64-darwin \
    --argstr nixpkgs-path /Users/ben/coding/nixpkgs/nixpkgs/.cache/nixpkgs-review/pr-366438-9/nixpkgs \
    --argstr nixpkgs-config-path /var/folders/mj/_03p5tkj30g1y7518fx_jydm0000gn/T/tmpfh76byaf.nix \
    --argstr attrs-path /Users/ben/coding/nixpkgs/nixpkgs/.cache/nixpkgs-review/pr-366438-9/attrs.nix \
    --nix-path 'nixpkgs=/Users/ben/coding/nixpkgs/nixpkgs/.cache/nixpkgs-review/pr-366438-9/nixpkgs nixpkgs-overlays=/var/folders/mj/_03p5tkj30g1y7518fx_jydm0000gn/T/tmp3844tuxd' \
    /Users/ben/coding/prs/nixpkgs-review/nixpkgs_review/nix/review-shell.nix

I suspect that this issue lies outside of this codebase, as searches for config.nix don't yield any matches, and the nom-shell CLI call does not reference such a path.
I had initially suspected this line, but applying .absolute() does not yield any change in behaviour.

A suspect would be this subprocess.run wrapper, as when env is None, the new subprocess will inherit the current process’ environment.

[SuperSandro2000]

FYI: you don't need to open an issue just for the process

[SuperSandro2000]

Specifically, for Python 3.11 and lower, mkdtemp will always return an absolute path when dir is None, and since Python 3.12, will always return an absolute pathname.

Do we even want to support older versions? But maybe for the time being a good idea anyway.

[bengsparks]

Versions of Python older than 3.12 are actually not supported; this file uses the type keyword to define a type alias, something that was first introduced in Python 3.12.

Anything older will fail with type System = str, SyntaxError: invalid syntax