title: The term 'System.DefaultWorkingDirectory' is not recognized in YAML pipeline
body: <p>I had a pipeline with PowerShell task that run some python script. It worked without any problems.
  After I convert my pipeline into YAML format to store it as code and got something like this (a part of whole yaml pipeline):</p>
  <pre><code>    variables:
        Build.SyncSources: false
        REPO_PATH_DA: '/asdfg/qwerty'
        REPO_PATH_DS: '/zxcvbn/tyuio'
        PIP_REPO_HOST: 'bbbb.nnnn.yyyy.com'
        PIP_REPO_URL: 'https://$(PIP_REPO_HOST)/api/pypi/pypi/simple'
        PIP_VENV_NAME: 'my_test_venv'
        SelectedBranch: ''
        WorkingDirectory: $(System.DefaultWorkingDirectory)
      
  …………………………………………….
  
      - task: PowerShell@1
        displayName: 'Install package'
        inputs:
          scriptType: inlineScript
          inlineScript: |
           .\$(PIP_VENV_NAME)\Scripts\activate
           python.exe -m pip install --index-url=$(PIP_REPO_URL) --trusted-host=$(PIP_REPO_HOST) mypackage
           python.exe -m pip install --index-url=$(PIP_REPO_URL) --trusted-host=$(PIP_REPO_HOST) $(WorkingDirectory)$(REPO_PATH_DA)\qwerty
  </code></pre>
  <p>And after I run this pipeline I get an error:</p>
  <pre><code>##[error]System.DefaultWorkingDirectory : The term 'System.DefaultWorkingDirectory' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
  </code></pre>
  <p>I tried to change the name of variable in format: $(env:System_DefaultWorkingDirectory) , but no success. I suppose that predefined variables are not passed into yaml pipeline. Do you have any ideas how to resolve it?</p>