ci: run rubocop on pull requests

[G-Rath]

Summary

Currently rubocop failures are not properly coming through on pull requests because the workflow only runs on push - this is why e.g. #1320 was able to slip through

Pull Request checklist

• Add/update test to cover these changes
• Update documentation
• Update CHANGELOG file

[Judahmeek]

@G-Rath According to my research (I believe this was one of my sources), the following configuration should eliminate any redundant job trigger, but also guarantee that CI runs on all PRs (including PRs from forks) and on pushes to master as well.

on:
  push:
    branches:
      - 'master'
  pull_request:

[G-Rath]

@Judahmeek yup you're right and that would be my preference but my change matches the other workflow - I'm happy to do a follow up PR changing both workflows to run only on pushes to master

[Judahmeek]

@G-Rath Fair enough. Although I would have just combined #1320, #1321, and said follow-up, I understand & respect your reasons for not doing so.

Btw, how did you notice my merge of #1320 so quickly?

[G-Rath]

Follow up PR is here: #1322

Btw, how did you notice my merge of #1320 so quickly?

I got a notification in my inbox and just... actioned it 😅