add manual dispatch trigger to GitHub actions

Author: timkpaine

.github/workflows/build.yml

@@ -6,8 +6,8 @@ on:
        - main
   pull_request:
   schedule:
-    # run on sunday nights
     - cron:  '0 0 * * 0'
+  workflow_dispatch:
 
 concurrency:
    group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}