_pipeline.py: Allow pulling strict artifacts in non-strict mode

Remotely cached artifacts matching the strict cache key take precedence
over locally cached artifacts matching only the weak cache key. However,
locally cached artifacts were excluded from the build plan, which means
that BuildStream never even checked whether the strict artifact is
available in the remote artifact cache.

This changes planning to keep cached elements in the build plan in
non-strict mode if a remote artifact cache is used.