title: Github Action workflow_run only on push to main branch
body: <p>I have a CI workflow that runs on PR and PUSH to main branch.</p>
  <pre><code>---
  name: CI
  
  on:
    push:
      branches:
        - main
    pull_request:
      types: [opened, synchronize, reopened]
  </code></pre>
  <p>I have another workflow I'd like to only run after CI is complete and conclusion is success but only when it's pushed to main branch.</p>
  <pre><code>---
  name: Build
  
  on:
    workflow_run:
      workflows: [&quot;CI&quot;]
      types:
        - completed
  jobs:
    build:
      name: Build
      runs-on: self-hosted
      if: ${{ github.event.workflow_run.conclusion == 'success' }}
  </code></pre>
  <p>It runs on both PR and push to main.  How do I get the Build workflow to only run on push to main?</p>