Run EE tests on PRs

Last week I accidentally merged a PR without running EE acceptance tests, and it was embarrassing because they failed. 😊

It looks like it's up to the maintainer to notice the EE test change, then push the PR to a local branch to run the EE acceptance tests. It feels error-prone and a little too much work.

@roidelapluie had a neat idea here to allow maintainers to trigger EE test runs from the PR directly, with a comment, but I don't think it was implemented.

Could we use the pull_request_review_comment webhook for this?

Nice to have: The workflow could auto-detect EE acceptance test changes on new PRs and block merging until someone triggers the acceptance test workflow. I'm not sure how feasible this is since workflow permissions on PRs can be restrictive.