chore: CI looks for nightly-testing-YYYY-MM-DD at Mathlib as either a…

… branch or tag

.github/workflows/pr-release.yml

@@ -126,7 +126,7 @@ jobs:
           if [ "$NIGHTLY_SHA" = "$MERGE_BASE_SHA" ]; then
             echo "The merge base of this PR coincides with the nightly release"
 
-            REMOTE_BRANCHES="$(git ls-remote -h https://github.com/leanprover-community/mathlib4.git nightly-testing-"$MOST_RECENT_NIGHTLY")"
+            REMOTE_BRANCHES="$(git ls-remote https://github.com/leanprover-community/mathlib4.git nightly-testing-"$MOST_RECENT_NIGHTLY")"
 
             if [[ -n "$REMOTE_BRANCHES" ]]; then
               echo "... and Mathlib has a 'nightly-testing-$MOST_RECENT_NIGHTLY' branch."
@@ -239,7 +239,7 @@ jobs:
           git config user.name "leanprover-community-mathlib4-bot"
           git config user.email "[email protected]"
 
-          if git branch -r | grep -q "nightly-testing-${MOST_RECENT_NIGHTLY}"; then
+          if git ls-remote --heads --tags origin | grep -q "refs/\(heads\|tags\)/nightly-testing-${MOST_RECENT_NIGHTLY}"; then
             BASE="nightly-testing-${MOST_RECENT_NIGHTLY}"
           else
             echo "This shouldn't be possible: couldn't find a 'nightly-testing-${MOST_RECENT_NIGHTLY}' branch at Mathlib. Falling back to 'nightly-testing'."