chore: when setting up Mathlib CI, make sure nightly-with-mathlib bra…

…nch has been fetched (#3834)

As reported on
[zulip](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/PR.20release.20failure/near/431281042).

.github/workflows/pr-release.yml

@@ -149,6 +149,7 @@ jobs:
             echo "but 'git merge-base origin/master HEAD' reported: $MERGE_BASE_SHA"
             git -C lean4.git log -10 origin/master
 
+            git -C lean4.git fetch origin nightly-with-mathlib 
             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