title: Azure DevOps: CI Pipeline executing after merge
body: <p>I have implemented a pipeline that will build and test the application once a Pull Request have been created. It works fine but it will run again once the PR is merged, and I only want it to run in the creation of a PR.</p>
  <p>The Yaml fine goes like:</p>
  <pre><code>trigger:
  - development
  
  pool:
    vmImage: ubuntu-latest
  
  variables:
    buildConfiguration: 'Release'
  
  steps:
  - task: UseDotNet@2
    displayName: 'Use .NET Core sdk 3.1.000'
    inputs:
      version: 3.1.x
  - task: DotNetCoreCLI@2
    displayName: 'Build the application'
    inputs:
      command: 'build'
      projects: 'example.csproj'
      configuration: 'Release'
  
  - task: DotNetCoreCLI@2
    displayName: 'Run unit tests'
    inputs:
      command: 'test'
      projects: 'example.csproj'
      configuration: 'Release'
  </code></pre>