CI: use a single invocation for apkbuild linting (MR 2472)

This replaces the current loop calling pmbootstrap lint once for each
package with a single invocation for increased performance.

Here are some measurements for linting all packages under main/.

Before this change:

    $ time .gitlab-ci/apkbuild-linting.py > /dev/null

    real    3m55,840s
    user    3m48,592s
    sys     0m16,913s

After this change but without pmbootstrap!2100:

    $ time .gitlab-ci/apkbuild-linting.py > /dev/null

    real    0m14,359s
    user    0m17,994s
    sys     0m6,488s

After this change with pmbootstrap!2100:

    $ time .gitlab-ci/apkbuild-linting.py > /dev/null

    real    0m6,411s
    user    0m13,334s
    sys     0m2,417s

Note that pmbootstrap!2100 is not required for that little
bit of extra performance but rather because it allows to differentiate
between linting errors for different packages in the output.

Depends: pmbootstrap!2100
Closes: #564