title: AzurePipelines: different values for `pipeline.startTime` in the same run of a pipeline
body: <p>I have a pipeline defined by the following two files. My problem is that the output of the <code>ShowDate</code> step is not the same in these two jobs. I would expect this because both are filled by <code>pipeline.startTime</code>. It seems that <code>pipeline.startTime</code> is evaluated in each job and the result is the start time of the job and not the pipeline itself.</p>
  <p>So my question is: how can I get the start time of the pipeline itself? Or what am I doing wrong here?</p>
  <p>When I run the pipeline I can see in the protocol of the job itself in section 'Job preparation parameters' that the right values are assigned to the variable <code>DateString</code> but this values are ignored.</p>
  <p>azure-pipelines.yml</p>
  <pre><code>stages:
  - stage: SomeStage    
    jobs:
    - template: job-template.yml
    - template: job-template.yml
  </code></pre>
  <p>and job-template.yml</p>
  <pre><code>jobs:
  - job: 
    variables:
      DateString: $[format('{0:yyyy}_{0:MM}_{0:dd}_{0:HH}{0:mm}{0:ss}', pipeline.startTime)]
    pool:
      name: SomePool
    steps:
    - script: echo $(DateString)
      displayName: ShowDate
    - task: PowerShell@2
      displayName: wait
      inputs:
        targetType: 'inline'
        script: 'Start-Sleep -Seconds 10'
  </code></pre>