[llvm] [Github] Fix security issues in libcxx-run-benchmarks.yml workflow (PR #158467)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

LGTM, thanks a lot.

FWIW, I would have much much rather gone through a PR with this workflow, but getting it to work was a matter of 10s of trial and error. I even created my own dummy repo to try out the workflow but there are a few things that could only be "tested" on the real monorepo (without incurring a lot of overhead in the feedback loop).

So the result is that I basically committed to main directly, which is a shame. I might just be unaware of better options, but as far as I can tell there is no good way to develop such workflows besides shipping directly to `main`. If there's a better way, we could probably document it cause I would have loved to get everything ready and then request your review before enabling the workflow.

https://github.com/llvm/llvm-project/pull/158467