Add a mechanism to rerun PR smoketests via setting a label ('smoketest-force-rerun')

If the smoketest action is re-run via github, then it will hit an exact cache match.
The usual behavior under those circumstances, is to skip building and running the tests.
Sometimes it is useful to force a rerun to re-test, e.g. to see if an issue is reproducible, or to check timing.

Detect if PR has label 'smoketest-force-rerun' and force PR to run even if cache-hit.
Also detect 'smoketest-ignore-cache' and ignore the cache completely if set, i.e. will perform a complete rebuild.