chore(ci): Build app only on push to main instead of on PR updates

[coreycb]

Fixes #11521

[wmontwe]

This removes the only pre-merge protection against breaking main.

The proposed push build is not an equivalent replacement. It runs only after the change has landed on main. At that point, CI can tell us that main is broken, but it cannot prevent the breakage. Every maintainer who updates their branch, starts new work, or opens another pull request can then encounter the failure until someone investigates and merges a fix or reverts the broken change.

This also transfers the burden from the author of the breaking change to the rest of the team. The failure may be discovered when the author is unavailable, and unrelated pull requests may become blocked or show failures caused by the broken base branch. That creates more disruption and investigation work than allowing CI to validate each update before merge.

The workflow already has:

    cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}                                                                                                                                                                                                    

so superseded runs for a pull request are cancelled.

In my opinion, reducing the number of runs does not justify removing the only pre-merge gate that assembles the applications and runs the unit tests and Android lint.

[coreycb]

I don't disagree. There was concern about the number of builds we're doing. I'll loop you into the conversation.