Provide repository info to gh command in GitHub action (#7491)

This fixes the following error in the GitHub action:

```
failed to run git: fatal: not a git repository (or any of the parent directories): .git
```

https://github.com/stylelint/stylelint/actions/runs/7609612527/job/20721207993#step:2:19

The failure reason is that the `gh` command tries reading the `.git` repository.
This change provides the repository info through an environment variable supported by `gh`.
(see also https://cli.github.com/manual/gh_help_environment)

.github/workflows/commenting-on-old-issues.yml

@@ -26,6 +26,7 @@ jobs:
             gh issue edit "${url}" --add-label 'status: ask to implement' --remove-label 'status: ready to implement'
           done
         env:
+          GH_REPO: ${{ github.repository }}
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           issue_comment: |-
             This issue is older than one month. Please ask before opening a pull request, as it may no longer be relevant.