This reverts commit d5cf10fb90cc3cea04a4ca241cce4cf2f800ea1d.

Requires internet access

Co-authored-by: Shavyn <Shavyn@users.noreply.github.com>

The owners check is not reproducible, because it depends on the state of
the NixOS org on GitHub. Owners can rename their accounts or they can
leave the organisation and access to Nixpkgs can be removed from teams.
All of this breaks the owners check for reasons unrelated to the PR at
hand.

This PR makes the check for the owners file conditionally required: Only
when the ci/OWNERS file is actually modified a failed check will block
merging the PR. When that's not the case, the check will still fail
visibily in the checklist, but the failure can be ignored.

This is especially relevant for the Merge Queue, which should not be
entirely blocked whenever any of these events happen.

Also, it allows passing the checks in a fork when testing, where the
owners check will *always* fail, because the respective teams and
members are never part of the "user org" that a fork is.

The required status checks should depend on exactly one workflow,
triggered via pull_request_target or merge_group. Anything that is
triggered by pull_request is for testing purposes of the workflows
themselves only.

Posting the status manually allows us to avoid the strange "skipped ==
success" logic and properly skip the `unlock` job for pull_request
events in the next commit.

This should be much easier to understand than the previous logic.