Heptapod CI: run workhorse jobs only when needed

This is another case of using `changes:`, close to what
upstream is doing. The new label will allow to force in cases
it should have run but didn't. Conversely, if it runs when it
should have not, there are no other consequences than wasted

branch : heptapod-stable
12 jobs for !327 with topic/heptapod-stable/ci-prebuilds-optional in 34 minutes and 34 seconds (queued for 2 seconds)
latest detached