enable manual triggering for main build (aws-observability#222)

*Description of changes:*
Enable manual triggering for main build so that `Run workflow` button
shows up under `Actions` tab for the workflow.

*Reference*

https://docs.github.com/en/actions/how-tos/managing-workflow-runs-and-deployments/managing-workflow-runs/manually-running-a-workflow

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choice.

Author: blairhyy-amazon

.github/workflows/main-build.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - main
       - "release/v*"
+  workflow_dispatch:
 
 env:
   AWS_DEFAULT_REGION: us-east-1