_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, #295 and #392