Update main_build.yml to be runnable on demand (#245)

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Author: thpierce

.github/workflows/main_build.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - main
       - "release/v*"
+  workflow_dispatch: # be able to run the workflow on demand
 env:
   AWS_DEFAULT_REGION: us-east-1
   STAGING_ECR_REGISTRY: 637423224110.dkr.ecr.us-east-1.amazonaws.com