Enable always writing cache to support hermetic build systems

I’d like to use actions/cache to cache my Bazel build state, which
includes dependencies that have been fetched, binaries and generated
code that have been built, and results for tests that have run. Bazel is
a hermetic build system, so the standard Bazel pattern is to always use
a single cache. Bazel will take care of invalidation at a fine-grained
level: if you only change one source file, it will only re-build and
re-test targets that depend on that source file.

Thus, the pattern that makes sense to me for Bazel projects is to always
fetch the cache and always store the cache. We can always fetch the
cache by using a constant cache key, but then the cache will never be
stored. Bazel doesn’t have a single package-lock.json-style file that
can be used as a cache key; it’s the combination of all build and source
files in the whole repository. We could key use the Git tree (or commit)
hash as a cache key, but this would lead to storing a mountain of
caches, too, which seems wasteful.

Ideally, the fetched cache would be taken from origin/master, but
really taking it from any recent commit should be fine, even if that
commit was in a broken or failing state.

On my repository, it takes 33 seconds to save the Bazel cache after a
successful job, but on a clean cache it takes 2 minutes to fetch remote
dependencies and 26 minutes to build all targets. I would be more than
happy to pay those 33 seconds every time if it would save half an hour
in the rest of the build!

For comparison, on Travis we achieve this by simply pointing to the
Bazel cache directory:
https://github.com/tensorflow/tensorboard/blob/1d1bd9a237fe23a3f2c31282ab44e7dfbcac717c/.travis.yml#L30-L32