_workspaces.py: Use file mtime for workspace cache keys instead of checksumming
The rationale for this is basically:
o A workspace is completely local
o A cache key for a workspace is used to determine if the workspace has changed or not
o Within reason, one should not expect buildstream to detect a change without a file's mtime having been modified
On my machine, it takes about 1 minute to checksum 11GB of WebKit checkout, that is annoyingly a long time. With this change, it takes only 5 seconds.
Not perfect, but it's a start.
This is related to issues #294 (closed), #295 and #392