CI: post gnu comment

[tertsdiepraam]

My attempt at: #4006

Heavily based on: https://securitylab.github.com/research/github-actions-preventing-pwn-requests/

It will not entirely work until it's merged, but I'm hoping to at least create the following zip file from the GnuTests workflow:

comment.zip
    - NR           <- the PR number saved to a file
    - result.txt   <- the actual results

This artifact can then be used to post the comment in the GnuComment workflow.

[tertsdiepraam]

Alright, that first step seemed to have worked! There is an artifact (though the result.txt is empty because the tests didn't change)

[sylvestre]

Let's see! :)