Exclude building PRs via *main* branch on cross repository builds [DBZ-6415]

Migrated from [DBZ-6415](https://issues.redhat.com/browse/DBZ-6415)

When the Debezium Server repository runs the CI cross repository build, it should checkout the core repository based on the contributor. See https://github.com/debezium/debezium-server/actions/runs/4758815019/jobs/8469248734?pr=19.

As per the current workflow the cross repository build would then look for *github.com/contributor/debezium.git* to check out and build rather than *github.com/debezium/debezium.git*, which didn't work on the above case.

Since the contributor created the PR using the *main* branch of his forked repository, when the workflow is checking the PR named branch ie *main* here it finds a match and runs the build based the PR branch instead of default branch.

Inspect the Debezium Server cross-repository build workflow and the linked GitHub Actions run. Reproduce the case where a contributor's fork uses the main branch, then verify that the workflow checks out and builds github.com/debezium/debezium.git rather than the contributor's fork. Done means the cross-repository build selects the upstream core repository for this case.

Written by the indexing model from the issue text.