title: need to set environment var for a back-ticks command
body: <p>how to pass an environment variable to a shell command that I execute using <code>Kernel#system</code> et al?</p>
  
  <p>say, I want to run</p>
  
  <pre><code>%x{git checkout -f}
  </code></pre>
  
  <p>but this command relies on the environment variable <code>$GIT_WORK_TREE</code>. how do I set it?</p>