ci: implement self-service ISO builds for every maintainer

This basically does the same as the PKGBUILDs repo CI, building an ISO if [build all]/[build isoname] is used as a commit message for master branch commits.

I'm planning to implement a [deploy all] trigger as well, to trigger a real release (uploading to SF/CF etc), therefore build has been used as keyword here.

Closes #2 (closed).