ci: Run e2e tests on pull_request

[samuelburnham]

Simplifies end2end CI to run on pull_request rather than issue_comment, i.e. the integration tests will automatically run on every PR commit rather than when !test is commented by a maintainer. This will decrease maintenance overhead while preserving security since we now use Buildjet and require approval to run CI.

Also fixes an issue where the issue_comment workflow wouldn't trigger the required status check, which seems to be an inherent GitHub Actions limitation.

Note

This should also be merged to the other feature branches for consistency.

[samuelburnham]

Closing as I forgot the pull_request trigger doesn't allow access to secrets 🤦