(fix) site_cache_dir() must return DVC_SITE_CACHE_DIR if it exists

[rgoya]

Fixes #10775

There was a regression introduced in #10603 where, when dealing with a change of behaviour in platformdirs, the fix effectively caused Linux (Unix) systems to ignore the DVC_SITE_CACHE_DIR environment variable.

This PR ensures that site_cache_dir() returns value of the DVC_SITE_CACHE_DIR variable is returned if it exists, and deals with the platformdirs issues if not.

---

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.05%. Comparing base (2431ec6) to head (86ea91c).
Report is 66 commits behind head on main.

Files with missing lines	Patch %	Lines
dvc/dirs.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10776      +/-   ##
==========================================
+ Coverage   90.68%   91.05%   +0.36%     
==========================================
  Files         504      504              
  Lines       39795    40007     +212     
  Branches     3141     3161      +20     
==========================================
+ Hits        36087    36427     +340     
+ Misses       3042     2951      -91     
+ Partials      666      629      -37     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[skshetry]

@rgoya, Thank you for contributing!

[rgoya]

@rgoya, Thank you for contributing!

👍

This had me scratching my head, it was working on my Macos laptop, but breaking in our CI pipeline (bazel test won't let me write to /var/tmp/). Needless to say, looking forward to the next release! :)

(Also, I have a related follow up bug and fix coming up)