Enable end2end tests on forked PRs

[samuelburnham]

This PR enables end-to-end tests to run on forked PRs with the following triggers:

• issue_comment: When a maintainer comments !test on a PR, it will trigger the end-to-end tests with full access to repo secrets. This enables the maintainer to carefully review PR comments before approving tests, without having to locally checkout and merge separately. This step is meant to give an early signal rather than be a required status check, so trivial or unrelated PRs can skip it.
• merge_group: This triggers the end-to-end tests whenever a PR is added to the merge queue, which this repo doesn't currently have but is used to great effect in https://github.com/lurk-lab/lurk-rs. Requiring the tests to pass when attempting to merge ensures correctness if the above PR comment action isn't run.

Successful runs:

• issue_comment: Test secret access ci-lab#16 and https://github.com/lurk-lab/ci-lab/actions/runs/6701350814/job/18208641508
• merge_group: https://github.com/lurk-lab/lurk-rs/actions/runs/6698173184

TODO:

• Factor out end-to-end test into separate action to remove code duplication

[storojs72]

!test

[samuelburnham]

@storojs72 The issue_comment trigger runs from the base branch, so unfortunately it's not possible to test until the action is merged to main and a new PR is opened. Please review the linked successful run carefully so we have confidence that this approach will work