[7.2.0] Wait for background tasks finished before releasing remoteCache. (#22500)

So that the last task won't try to shutdown the remoteCache, otherwise,
might cause deadlocks.

Fixes https://github.com/bazelbuild/bazel/issues/21568.

PiperOrigin-RevId: 635746896
Change-Id: Ic321216dd0bda05ef1ad18367c8ebbcd1a1d272c

Commit
https://github.com/bazelbuild/bazel/commit/a804fb1748ce767d98f8b7a40e660ed22576e30e



Co-authored-by: Googler <chiwang@google.com>