Benchmark PR job failing due to permissions

[kivikakk]

See e.g. https://github.com/kivikakk/comrak/actions/runs/5958432069/job/16162659993?pr=337.

Run mshick/add-pr-comment@v2
  with:
    message-path: bench-output.md
    message-id: add-pr-comment
    refresh-message-position: false
    repo-owner: kivikakk
    repo-name: comrak
    repo-token: ***
    allow-repeats: false
    status: success
  env:
    CARGO_INCREMENTAL: 0
    CARGO_TERM_COLOR: always
Error: Resource not accessible by integration

This "Resource not accessible by integration" message has been coming up on a few projects lately where it didn't before; looks like Actions has changed their permissions model a bit.

[digitalmoksha]

In particular it looks like a PR from a fork doesn't have the correct permissions.

mshick/add-pr-comment#67 and https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token

[kivikakk]

Possible solution: https://github.com/mshick/add-pr-comment#proxy-for-fork-based-prs