set retention days to 7 days

Author: stefafafan

.github/workflows/branch-preview.yaml

@@ -58,6 +58,7 @@ jobs:
       uses: actions/upload-pages-artifact@v3
       with:
         path: ./output/
+        retention-days: 7 # 1週間くらい残しておく
 
   deploy:
     permissions: