Ensure reporter action doesn't fail on PRs from forked repositories

[andrewiggins]

See benchmark run here for an example: https://github.com/Polymer/lit-html/pull/1322/checks

[andrewiggins]

Snapshot of sample failing logs (in case github cleans them up):

[andrewiggins]

To fix this, let's add support for the workflow_run event trigger. Benchmarks should be run in the pull_request event and the result json files uploaded as artifacts. When it finishes, it should trigger another workflow that consumes the JSON files as an artifact and runs this action to create/update the comment.

Note, the workflow_run should run for the requested and completed events so it can report running status on comments when the benchmark run starts.

Blog posts to reference:

• https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/
• https://securitylab.github.com/research/github-actions-preventing-pwn-requests/

Other things to consider:

• Perhaps add a pr_number input to allow people to manually specify the pr that workflow_run should update.
• If no pr_number is available, then we could try to guess which pr from one of the properties of context should be used. Perhaps pick the most recently updated open PR?