Restore write permission before clearing staging directory

Shared snapshots don't have write permissions on them to prevent accidental writes into them. If Gitaly terminated uncleanly and didn't clean up all of the shared snapshots, their directories would still be missing the write permission and fail the RemoveAll call when we're attempting to clear the staging directory on starting up.

Restore the write permission in the staging directory so read-only shared snapshots don't fail the deletion.