Enabling git cache with GIT_CACHE_PATH

[DanKegel]

Has anybody else had luck with GIT_CACHE_PATH?

I'd like to speed up the git part of cef builds (and also reduce load on the git server and our internet connection).
With Electron, setting GIT_CACHE_PATH to, say, $HOME/foo.dir works well.
With cef (at least with cef 3497, on the one mac I've tried it on), it only halfway works; the cache is
partially populated, and build fails fairly early because ```git checkout --force refs/tags/69.0.3497.32``` fails to find that tag.