CI is not triggered for pull requests

[AkihiroSuda]

img/.github/workflows/make-all.yml

Line 1 in 54d0ca9

on: push

We should enable CI for pull requests as well

[nathanblair]

Hi @AkihiroSuda !

I would love to take a shot at this but would like to know more about the desired behaviors.

To narrow it down:

• Are you wanting all PRs against just the default shared branch (master, in this case)? Or every PR against every branch in the repository?
  • I'm not sure what the project expects for maintaining multiple branches on this repo, so just want to check. Currently there is only one other branch existing.
• Which workflows exactly do you have in mind to run on PRs? Just the make-all workflow? Or should make-image also be included?
• Presumably the code that is built should be the code on the head branch?
  • I'm sure this sounds like a silly question but I don't like to assume anything if I can help it!