Speed up build_cache process when not needing to build cache

[mcbarton]

This should essentially allow workflow runs to skip over the build_cache matrix stage when the cache exists and doesn't need to be built.

[mcbarton]

@vgvassilev This PR takes waiting for the build_cache stage running from 3 1/2 minute to 21 seconds, when a cache already exists, allowing PRs to run faster.

[vgvassilev]

Nice!