build_changed_aports.py: allow skipping build and skipping package count sanity check (CI)

build_changed_aports.py will fail in three cases:

1. building a package failed
2. sanity check: too many packages (more than 10) have changed
3. execution time reaches one hour

It would be nice if we could override2., if a lot small packages have changed (e.g. we changed a lot of device packages).

And it would also be nice if we could skip build_changed_aports.py altogether to 3., but without disabling all CI checks as it would be the case with writing [skip ci] in the commit message.

Ideas for strings to check against:

• [pmaports-build-ci: ignore count] (maybe we can find a better string that is more descriptive?)
• [pmaports-build-ci: skip]
• [pmaports-build-ci: no version bump] (or something like that, see the idea below)