Cache keys not printed correctly with workspaced elements

We have some bad bugs with how cache keys are reported for workspaced elements, and worse, sometimes they are never printed at all.

For the latter part, it should probably be handled by #252 (closed)

Some scenarios with bugs:

Building workspaced element

• Open a workspace on an element
• Build that element
  • Notice that in the pipeline summary before building, the workspace has a cache key, this is the wrong cache key
  • Notice that the element starts out with cache key printed as [????????]
  • Notice that the real cache key which is resolved after the build completes, is never printed in the master log
• Build the element again
  • Now the cache key in the pipeline summary is correct

Running a shell on a workspaced element

• Open a workspace on an element
  • Now the element must be built at least once for it to have a cache key
• Run bst shell --build <workspaced-element.bst>
  • While staging dependencies for the workspaced element, a cache key is printed; as BuildStream cannot know the cache key at this time, it has to be the wrong cache key, instead it should be [????????]