element.py: Fix buildable check in non-strict mode

Ensure that the strong cache key of each build dependency is available
before an element is built. Otherwise the strong cache key of the
element cannot be calculated and caching the artifact produces an
AssertionError.

In non-strict mode an element's strong cache key may not be available
yet even though an artifact is available in the local cache. This can
happen if the pull job is still pending as the remote cache may have an
artifact that matches the strict cache key, which is preferred over a
locally cached artifact with a weak cache key match.

Fixes #383.