so:libprotobuf.so.15 does not exist anymore in Alpine edge

When cross compiling, the kernel scripts would be in the native arch
again. That was already fixed in !1234, however the mime type of the
binaries has changed, so we couldn't detect them properly anymore
(#1659).

This commit fixes it again, and also adds a checks so this won't
silently fail in the future.

[skip ci] because this would not run through anyway (building these
packages takes too long). Fixes #1659.

Update happened upstream in Alpine edge:
https://github.com/alpinelinux/aports/pull/4983

Copy all test cases from the pmbootstrap repository, that are actually
testing the aports. They were all adjusted to work nicely in this
repository, together with a brand new set of gitlab-ci configs.

This also includes the changes from this merge request, that had a
better detection of changed packages:
<https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1621>

All aports are from the pmbootstrap repo, which is licensed under
GPLv3. To learn more about the split, see:
<https://gitlab.com/postmarketOS/pmbootstrap/issues/383>

[skip ci]