Setup ruleset for act tests

[xnyo]

Act tests currently don't block PRs. We should set up a ruleset to force them to pass before merging PRs.

Act tests are run only when relevant actions/workflows/testdata are edited:

plugin-ci-workflows/.github/workflows/pr-checks-test-ci.yml

Lines 3 to 10 in e739fbe

	on:
	pull_request:
	branches:
	- main
	paths:
	- "tests/**"
	- ".github/workflows/pr-checks-test-ci.yml"
	- ".github/workflows/ci.yml"

We should instead:

• Always run the workflow
• Run act (slow) only when the relevant change, detect the relevant files via https://github.com/marketplace/actions/changed-files
• If no relevant files are updated, short circuit and exit with success, so the workflow succeeds (and the ruleset passed) but act tests (slow) are not run