Don't run ui-test-x86-{nightly,beta} tasks on pull requests

[jcristau]

This seems to be what was intended all along, but they were picking up run-on-tasks-for from job-defaults.

These tasks will now only run on github-push graphs, with ui-test-x86-debug also running on PRs.

Pull Request checklist

• [ ] Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• [ ] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

QA

• [ ] QA Needed

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to build-focus-debug or build-klar-debug for changes targeting Klar,
3. click View task in Taskcluster,
4. click the Artifacts row,
5. click to download any of the apks listed here which use an appropriate name for each CPU architecture.

GitHub Automation

Used by GitHub Actions.

[jcristau]

@rpappalax @AaronMT if this is not ok let me know and I'll replace this with another PR to remove the run-on-tasks-for setting from the x86-nightly and x86-beta jobs since right now they're super confusing.