add workflow_dispatch in scala-steward.yml

Author: xuwei-k

.github/workflows/scala-steward.yml

@@ -4,6 +4,7 @@ on:
     branches: [ master ]
   schedule:
   - cron: '0 2 * * *'
+  workflow_dispatch:
 jobs:
   scala-steward:
     runs-on: ubuntu-latest