CI: add check for unreferenced files in aports

Requires https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/1990

Fixes #496 (closed)

See also !1730 (closed)

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EDIT by ollieparanoid:

TODO:

• [ ] make sure that all packages build (@z3ntu, did you test this already?)
  • you wrote that you're pretty certain that this doesn't break any builds. I agree, after all we are just removing unused files and CI verifies that the checksums of the sources match, even with [ci:skip-build]. So let's ship it.