Shared layer.tar caches

[rcowsill]

Following on from #97, once issue #75 is fixed it will be possible to share the cached layer.tar files across multiple workflows.

This can be done by changing the cache keys of the layer.tar files to not include root cache hash. Multiple root caches (with different keys) could share layer.tar files rather than needing to upload their own copies.

The restrictions on cache sharing between branches would still be present, but layers cached by the default branch would be available to workflows in all branches.