CI: avoid duplicate runs for secondary branches on main repo

[EZoni]

The fix introduced in #5308 was not correct for Azure pipelines.

In GitHub Actions we trigger a run on the push event only for the development branch.

The Azure equivalent of that is triggering a run on the trigger event only for the development branch. However, since the trigger event was completely absent from the Azure pipeline file (that is, the default setup was being used), I had erroneously added the filter branch to the pr event instead, unlike what I did for GitHub actions where the push was exposed in the YAML files.

This was originally aimed at avoiding duplicate runs for "individual CI" when pre-commit opens a pull request by pushing to a secondary branch pre-commit-ci-update-config in the main repo (instead of a fork).

The new setup is tested in #5393, where I copied these changes and where one can see that a commit pushed to that PR does not trigger an "individual CI" Azure pipeline anymore, but only a "PR automated" one.

Hopefully this is correct for the merge commits that get pushed to development once a PR is closed, but we'll be able to test this only after merging a PR.

[EZoni]

Before this PR, after a push to, e.g., #5393:

After this PR, after a push to, e.g., #5393: