WIP: [ci] fix valgrind workflow

[jameslamb]

The valgrind CI job is failing like this:

/__w/_temp/a09e15f1-9453-4607-9d54-66e1c2404ea4.sh: 6: comment+=https://github.com/microsoft/LightGBM/actions/runs/13159252384: not found

(build link)

This attempts to fix that.

Notes for Reviewers

I first thought that the root cause was that GitHub Actions falls back to sh is bash is not found in PATH: https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#defaultsrunshell

But wch1/r-debug (the container this runs in) has bash 🤔

[jameslamb]

/gha run r-valgrind

Status: failure ❌.

[jameslamb]

/gha run r-valgrind

Status: failure ❌.

[jameslamb]

Still seeing an error about += in logs even after removing it here: https://github.com/microsoft/LightGBM/actions/runs/13191763540/job/36825886728#step:6:48

I forgot that for these comment-triggered workflows, the GitHub Actions config is sourced from master. We'll have to merge these changes to test them.

Now that this PR has a failing run of valgrind, it'll be blocked from normal (non-admin) merging. I'll open a new PR.