Last successful workspace build can go missing

If the user removes/wipes their artifact cache, or if a cleanup procedure automatically deletes the artifact because it is not marked as required in the current session, the related workspace metadata is not updated as a result.

Since we do not handle the case where the last successful build artifact has gone missing, we run into bugs which give the user no obvious recourse - the only way out is to close the related workspace, however the user doesn't really know this.

This currently results in the incorrectly reported error message (as described in #1016):

[00:00:00][????????][build:sdk/glib.bst                  ] FAILURE Attempt to access unavailable artifact: [Errno 2] No such file or directory: '/home/tristan/.cache/buildstream/artifacts/cas/refs/heads/gnome/sdk-glib/d371839c675f4be8c70f9f212093531699e77baf6500e8e56d8dbd7270607068'

Marking this critical as it results in a situation where the user is stuck and cannot move forward without in depth knowledge of BuildStream.

The workaround currently is to close the related workspace.