ci: add workflow_dispatch trigger to cron workflow

Author: ttskch

.github/workflows/cron.yaml

@@ -3,6 +3,7 @@ name: cron
 on:
   schedule:
     - cron: "0 15 * * *" # 00:00 JST
+  workflow_dispatch:
 
 jobs:
   # https://zenn.dev/snowcait/articles/903d86d668fcb7