(fix) DVC_SITE_CACHE_DIR environment variable should override core.site_cache_dir

[rgoya]

Fixes #10777

Currently: Repo.site_cache_dir() in dvc/repo/__init__.py selects first the configuration read in the config file, and if it does not exist it relies on dirs.site_cache_dir() to pick a value.

The documentation states that DVC_DITE_CACHE_DIR should override this configuration, therefore the order of priority for selecting the site_cache_dir is:

1. DVC_SITE_CACHE_DIR
2. core.site_cache_dir
3. platformdirs selection

For this, dirs.site_cache_dir() needs to know the three options.

This PR:

• enables dirs.site_cache_dir() to receive a configuration option so it can make the right choice between the three values;
• modifies Repo.site_cache_dir() to send core.site_cache_dir instead of picking itself; and
• enables tests for the proper return values.

---

• ❗ 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

All modified and coverable lines are covered by tests ✅

Project coverage is 91.06%. Comparing base (2431ec6) to head (a9f9ea7).
Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10778      +/-   ##
==========================================
+ Coverage   90.68%   91.06%   +0.38%     
==========================================
  Files         504      504              
  Lines       39795    40016     +221     
  Branches     3141     3162      +21     
==========================================
+ Hits        36087    36441     +354     
+ Misses       3042     2948      -94     
+ Partials      666      627      -39     

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