Update build.yaml

Author: bbartholdy

.github/workflows/build.yaml

@@ -1,6 +1,7 @@
 # Workflow derived from https://github.com/r-lib/actions/tree/master/examples
 # Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
 on:
+  workflow_dispatch:
   push:
     branches: 
       - main