Don't run buf-ci push job in forks

[anuraaga]

Currently this workflow always fails on a push event in the fork repo, like

https://github.com/anuraaga/protovalidate/actions/runs/32110482296

This preserves handling push events in this repo while allowing contributor forks to not go red. Note, personally it's a bit confusing the same workflow runs twice on PRs for branches in this repo - I think reworking to restricting push to main and configuring the job to format/lint/push appropriately could be cleaner but took the easy approach for now.

[emcfarlane]

This is odd, it shouldn't be running on forks.

[anuraaga]

Since the push event doesn't specify a branch (intentionally FWICT, to check PRs against the BSR), it gets executed from the push in the fork itself (can't send a PR without pushing somewhere). So the repository check takes care of making sure it's not a fork - most repos only run on main push and have less of an issue.