CI/GnuComment: enhance PR commenting logic to update existing comments

[xtqqczze]

Closes #11761

Summary

Update GNU testsuite comment workflow to edit a single bot comment instead of creating new ones on each run.

Key change

• Introduce a hidden marker (<!-- gnu-comment-bot -->) to identify bot-generated comments.
• Only comments with this marker are updated, so this applies to new comments going forward (existing comments remain unchanged).

[xtqqczze]

@sylvestre How can we test this?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[sylvestre]

let's see what happens

[cakebaker]

It looks like there is a bug (see https://github.com/uutils/coreutils/actions/runs/24334867494/job/71049096034#step:4:44):

Error: ENOENT: no such file or directory, open './result.txt'
    at Object.openSync (node:fs:560:18)
    at Object.readFileSync (node:fs:444:35)
    at eval (eval at callAsyncFunction (/home/runner/work/_actions/actions/github-script/v9/dist/index.js:64949:16), <anonymous>:5:20)
    at callAsyncFunction (/home/runner/work/_actions/actions/github-script/v9/dist/index.js:64950:12)
    at main (/home/runner/work/_actions/actions/github-script/v9/dist/index.js:65100:26)
    at /home/runner/work/_actions/actions/github-script/v9/dist/index.js:65069:1
    at /home/runner/work/_actions/actions/github-script/v9/dist/index.js:65147:3
    at Object.<anonymous> (/home/runner/work/_actions/actions/github-script/v9/dist/index.js:65150:12)
    at Module._compile (node:internal/modules/cjs/loader:1812:14)
    at Object..js (node:internal/modules/cjs/loader:1943:10) {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: './result.txt'
}
Error: Unhandled error: Error: ENOENT: no such file or directory, open './result.txt'

[xtqqczze]

Error: Unhandled error: Error: ENOENT: no such file or directory, open './result.txt'

I don't understand why this fails now, but let's add something like:

if (!fs.existsSync('./result.txt') || !fs.existsSync('./NR')) {
  console.log('Files in workspace:', fs.readdirSync('.'));
  throw new Error('Missing result.txt or NR from artifact');
}

[xtqqczze]

#11793

[xtqqczze]

Looks like it's working now: #11330 (comment)