title: vsts-npm-auth can't get authentication token on VSTS build
body: <p>I'm attempting to use <a href="https://www.npmjs.com/package/vsts-npm-auth" rel="nofollow noreferrer">vsts-npm-auth</a> to get the authentication token for our VSTS package repository. On my development machine I can run the commands</p>
  
  <p><code>npm install -g vsts-npm-auth
  vsts-npm-auth -config path-to-my\.npmrc</code></p>
  
  <p>and it succeeds in providing me with an authentication token. I'm now trying to recreate this as a build step on VSTS, so I create the powershell script <code>auth-vsts.ps1</code></p>
  
  <pre><code>$npmrcFile = "$PSScriptRoot\path-to-my\.npmrc";
  npm install -g vsts-npm-auth;
  vsts-npm-auth -config $npmrcFile;
  </code></pre>
  
  <p>and add it as a powershell task. However, the task fails as follows</p>
  
  <pre><code>2017-05-30T09:37:41.1082686Z ##[section]Starting: auth-vsts
  2017-05-30T09:37:41.1092712Z ==============================================================================
  2017-05-30T09:37:41.1092712Z Task         : PowerShell
  2017-05-30T09:37:41.1092712Z Description  : Run a PowerShell script
  2017-05-30T09:37:41.1092712Z Version      : 1.2.3
  2017-05-30T09:37:41.1092712Z Author       : Microsoft Corporation
  2017-05-30T09:37:41.1092712Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkID=613736)
  2017-05-30T09:37:41.1092712Z ==============================================================================
  2017-05-30T09:37:41.1112679Z ##[command]. 'd:\a\1\s\auth-vsts.ps1' 
  2017-05-30T09:37:47.3792461Z C:\NPM\Modules\vsts-npm-auth -&gt; C:\NPM\Modules\node_modules\vsts-npm-auth\bin\vsts-npm-auth.exe
  2017-05-30T09:37:47.3792461Z C:\NPM\Modules
  2017-05-30T09:37:47.3802239Z `-- vsts-npm-auth@0.25.0 
  2017-05-30T09:37:47.3802239Z 
  2017-05-30T09:37:47.3802239Z 
  2017-05-30T09:37:47.3802239Z vsts-npm-auth v0.25.0.0 
  2017-05-30T09:37:47.3802239Z -----------------------
  2017-05-30T09:37:47.3802239Z Creating npmrcFile. Path: D:\a\1\s\.npmrc
  2017-05-30T09:37:47.3802239Z Getting new credentials for source:https://our-domain/_packaging/SharedLib/npm/registry/, scope:vso.packaging_write vso.drop_write
  2017-05-30T09:37:49.8729702Z Caught exception: The prompt option is invalid because the process is not interactive.
  2017-05-30T09:37:49.8729702Z Parameter name: PromptType
  2017-05-30T09:37:49.8729702Z Caught exception: The prompt option is invalid because the process is not interactive.
  2017-05-30T09:37:49.8729702Z Parameter name: PromptType
  2017-05-30T09:37:49.8729702Z Couldn't get an authentication token for //our-domain/_packaging/SharedLib/npm/registry/:_authToken.
  2017-05-30T09:37:50.1769711Z ##[error]Process completed with exit code 1.
  2017-05-30T09:37:50.1809715Z ##[section]Finishing: auth-vsts
  </code></pre>
  
  <p>The error gives no indication as to why it can't obtain the credentials. Any ideas why this might be?</p>