invalidate-cache: true doesn't work on self-hosted runners

Self-hosted runners persist a lot more across runs vs. GitHub-provided runners. Currently the code just skips cache restoration to the runner's local FS when `invalidate-cache: true` is set. However, the cache is usually already present in its restored location on self-hosted runners. So an additional check is needed to ignore that w/ `invalidate-cache: true`. I'll work up a PR.