build: prevent github actions run when pushing master branch on forked cockroach db repo

If I push the master branch on my fork of cockroach, the[ github actions suite](https://github.com/msbutler/cockroach/actions/runs/9958971847) seems to run and fail -- note it was running for about a day as well. This seems like a bug.

Jira issue: CRDB-40436

Start with the linked GitHub Actions run and inspect the workflow triggered by a push to master in a fork of cockroachdb/cockroach. Reproduce the fork push behavior and identify why the suite runs and remains active; done means the unwanted run is prevented without changing the intended workflow behavior for the main repository.

Written by the indexing model from the issue text.