chore: avoid duplicate actions runs

[wincent]

As explained here:

• https://wildwolf.name/github-actions-how-to-avoid-running-the-same-workflow-multiple-times/

Before:

• If I pushed to a branch, without making a PR, actions would run once.
• If I pushed to a branch, then opened a PR, actions would run twice.
• If somebody else opened a PR (from a fork), actions would run once.

After:

• If I push to a branch, without making a PR, actions will run once.
• If I push to a branch, then open a PR, actions will run once (ie. due to the push, not the PR).
• If somebody else opens a PR (from a fork), actions will run once.

[wincent]

Evidence that this works (note the entries labeled with "skipped"):