6 - Avoid double runs for gh actions

[aedwardg]

Closes #6

This change should ensure the following happens:

• On any push to branches within this repo, the format-and-lint action runs.
• The format-and-lint action only runs on pull_request events when that pull request is from a forked repo, ensuring that we don't have double runs on every single push.

For this PR, you can see that the pull_request job is skipped

As can be seen in #12, only the pull_request events trigger the action when it is from a forked repo

[Victoria-DR]

LGTM