element.py: Cache the result of checking whether an artifact is cached weakly

Normally we'd only need it in the case of scheduling a weakly cached build,
but to allow caching of failed builds we need to be able to distinguish
between cached successes and cached failures
for both strong and weak cache keys.

To allow other cache lookup codepaths to look up via the weak key
requires changes through the call stack to consult which key to use,
and cache invalidation of the saved state when it changes.