Benchmark workflow: fix behavior with pull requests from forks

[dagood]

POST /repos/golang-fips/openssl/issues/436/comments - 403 with id 3818:30180C:334A19E:D0FC113:69ED585F in 118ms
RequestError [HttpError]: Resource not accessible by integration - https://docs.github.com/rest/issues/comments#create-an-issue-comment

https://github.com/golang-fips/openssl/actions/runs/24942838372/job/73042027650#step:7:44
(#436 (comment))

I'm not totally sure that the cause is the PR being created from a fork (I don't see this mentioned simply in the docs), but it's the most obvious difference and it would make sense to me.

/cc @michelle-clayton-work

[gdams]

This will be to do with the whole pull_request vs pull_request_target issue. Basically PR's generated from forks don't have GITHUB_TOKEN permissions so they need to be run via a pull_request_target workflow but this is dangerous teritory so we'd need to be cautious.

[dagood]

Is it possible to make the workflow succeed (if no regressions) and point out why it didn't post a comment clearly in the step that would have normally done that? That shouldn't need any more permission and should fix it, even if it's a degraded experience.