Don't cache the whole base directory; instead independently cache:
- the build output directory
- the ccache directory
This allow completely avoiding the need for building base if a
cache (keyed on the relevant parts of the git tree for base) is
successfully restored, as well as reducing the amount of cached
data.
Additionally, ensure caches are isolated by branch name (but allow
falling back to a "master" cache for restore): we don't want PRs
using each others' caches.
