title: Yarn not install git private repositories
body: <p>I am using yarn but it is giving an below error for private git repositories. npm installed this repo successfully.</p>
  
  <pre><code>yarn install v1.2.1
  info No lockfile found.
  [1/4]   Resolving packages...
  error Command failed.
  Exit code: 128
  Command: git
  Arguments: ls-remote --tags --heads ssh://git@github.com/test/testconfigs.git
  Directory: /Users/admin/Projects/Demo/myProject
  Output:
  Permission denied (publickey).
  fatal: Could not read from remote repository.
  </code></pre>