nightly: use target_commitish for previous SHA detection instead of body parsing

.gitea/workflows/nightly.yml

@@ -42,10 +42,11 @@ jobs:
           PREV=$(curl -s -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" \
             "$API/repos/${GITHUB_REPOSITORY}/releases/tags/nightly" \
             | python3 -c "
-          import json,sys,re
+          import json,sys
           try:
-              m = re.search(r'commit: ([0-9a-f]{40})', json.load(sys.stdin).get('body',''))
-              print(m.group(1) if m else '')
+              r = json.load(sys.stdin)
+              sha = r.get('target_commitish','')
+              print(sha if len(sha) == 40 else '')
           except Exception:
               print('')")
           if [ "$PREV" = "$SHA" ]; then