Prefer pull_request over push for same-commit tarball builds

Buildbot treats push and pull_request events independently. The scheduler
can trigger tarball-docker for both event types, while push events are only
considered when the upstream branch matches a configured pattern such as
bb-* or st-*.

When a pull request is opened from a branch in the upstream repository, the
source branch may also match the push-event filter. In that case, two
tarball builds can be scheduled for the same commit: one from the push
event and one from the pull_request event.

This patch cancels the tarball triggered by the push event and keeps only
the pull_request build, since that is the one relevant for branch
protection and GitHub Checks.

Note that *-pkgtest builds cannot be run meaningfully from both a pull
request and the corresponding upstream branch at the same time. Any pkgtest
should therefore be completed before opening the pull request.