oxide ci should run on the main branch

[proppy]

It doesn't seem that the oxide ci is ever run on main, see:
https://github.com/google/CFU-Playground/actions/workflows/oxide.yml?query=branch%3Amain++

As discussed by @tcal-x on #810 (comment), it would be nice to add a push event trigger:

to make it easier to find the ongoing failures.