Use git log %s (subject) for commit_message env var

%B emits the full body. Multi-line commit messages corrupt
$GITHUB_ENV (expects KEY=VALUE per line). Subject is sufficient
for the downstream check against 'Apply automatic changes'.

Author: Max Winterstein

.github/workflows/update_shfmt_checksums.yml

@@ -22,7 +22,8 @@ jobs:
 
       - name: Get last commit message
         id: get_commit_message
-        run: echo "commit_message=$(git log -1 --pretty=%B)" >> $GITHUB_ENV
+        # %s = subject line only; %B (full body) breaks $GITHUB_ENV on multi-line messages.
+        run: echo "commit_message=$(git log -1 --pretty=%s)" >> "$GITHUB_ENV"
 
       - name: Set up Python
         uses: actions/setup-python@v6