- define deny list of files when modified not to run GHA
- specRitfy OS to be ubuntu-20.04 (is there a debian at all?)
- only use perl 5.26
While researching I stumbled over mac.osx as an OS one can specify. If anybody
knows whether this is really macosx (license?) please let me know. That
would be a great addition for CI. (Couldn't find BSD yet...)
See #1920