CI: Improve performance when linting lots of APKBUILDs

The "aport-lint" step in CI is really slow if we are linting lots of APKBUILDs, see e.g. https://gitlab.postmarketos.org/postmarketos/pmaports/-/jobs/526586969

I suspect this happens because we start pmbootstrap separately for each APKBUILD instead of linting multiple ones in one invocation. Maybe someone wants to investigate if there is a way to optimize it?

Alternatively it would already help if linting issues were printed immediately once they are found. (So you can see some progress...) At the moment they are only printed once everything has been linted.