[Request] Cache downloaded sources

[xiota]

Some packages have very large source downloads. Caching sources would save significant bandwidth on rebuilds.

Example packages include chromium-based browsers, firefox-based browsers, and many -git packages.

Instead of entirely clearing cache, problem files could be deleted:

• When the hash changes. Check .SRCINFO
• The git repo is out of sync. Compare .SRCINFO with .git/config

Full cache clear could be triggered manually. Since each builder may have its own cache, maybe a variable in .CI/config could store a timestamp that forces a cache clear if any files/folders are older.