Rollup merge of #138223 - Kobzol:fix-post-merge, r=marcoieni

Fix post-merge workflow

I forgot that `actions/checkout` only checks out a single commit by default. I also forgot to set the environment variable required for the `gh` CLI commands.

I did a few more tests on my fork and hopefully now it should work properly. I also tested it with fake rollup PRs and the comment was sent only to the merged rollup, as it should be.

r? `@marcoieni`

Author: matthiaskrgr