Restore the mono_repo self validate

[natebosch]

We are temporarily pinning to ubuntu-20.04 instead of ubuntu-latest. mono_repo does not support anything than ubuntu-latest, so the change are being made manually and the self validate step is being disabled. We should go back to normal once github actions ubuntu-latest has firefox again, which should happen this week: https://github.com/actions/runner-images/issues/6399#issuecomment-1328761049