Don't run nightly/beta UI tests on all pushes to the repo

[jcristau]

We're only interested in these tasks on-push for the "level 3" branches, i.e. main and releases_v*; development branches shouldn't run these tasks.

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• [ ] 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.

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• [ ] QA Needed

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[jcristau]

Sorry I think I need to look at this more.

[AaronMT]

@jcristau I'm likely going ahead with the same UI test job environment change / job rename on Focus that I did on Fenix. If you'd like to proceed with this before-hand as it touches taskcluster/ci/ui-test/kind.yml