chore: fix rebase suggestion for Mathlib CI (#3701)

Previously we were suggesting rebasing onto the most recently nightly in
the branches history, but that is incorrect and we should *always*
suggest rebasing on `origin/nightly-with-mathlib`.

---------

Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>

.github/workflows/pr-release.yml

@@ -149,7 +149,8 @@ jobs:
             echo "but 'git merge-base origin/master HEAD' reported: $MERGE_BASE_SHA"
             git -C lean4.git log -10 origin/master
 
-            MESSAGE="- ❗ Std/Mathlib CI will not be attempted unless your PR branches off the \`nightly-with-mathlib\` branch. Try \`git rebase $MERGE_BASE_SHA --onto $NIGHTLY_SHA\`."
+            NIGHTLY_WITH_MATHLIB_SHA="$(git -C lean4.git rev-parse "nightly-with-mathlib")"
+            MESSAGE="- ❗ Std/Mathlib CI will not be attempted unless your PR branches off the \`nightly-with-mathlib\` branch. Try \`git rebase $MERGE_BASE_SHA --onto $NIGHTLY_WITH_MATHLIB_SHA\`."
           fi
 
           if [[ -n "$MESSAGE" ]]; then