PRs can still be merged with failing workflows

[iainlane]

We had this failing run but we still merged #147 and it broke users.

It's because this check isn't required. Conditional checks can't simply be made required, or else they will block pull requests which don't change the files they trigger on.

Possible fixes:

1. Make checks unconditional, just run them always
2. Move the conditionality to runtime using a "changed files" type of action
3. Use our deployment of https://github.com/palantir/policy-bot and make these checks conditional if triggered by writing a .policy.yml containing rules that replicate the file triggers, or using a generator (we have one internally in deployment_tools - we could open source that by moving it to this repo).

[Elfo404]

Option 3 should not be viable due to #497 (comment)