use pmbootstrap v3 in gitlab CI

pmaports CI should use pmbootstrap v3, this is also a great way to put pmb3 through more testing.

One way to roll this out and gain confidence in pmb3 is to duplicate all pipelines, use pmb2 (like we do today) and run the same jobs with pmb3 in parallel. Jobs using pmb3 can fail and not block merging, at least until we're ready to replace pmb2. Update: I think we should just move to pmb v3, as @calebccff suggested elsewhere, and not try to have both v2 and v3 running in CI

I added this to milestone %initial systemd support because I believe that we need pmb 3 in CI before we merge the systemd staging branch