title: How do you update your "HOMEBREW_TEMP" environmental variable?
body: <p>I'm getting the following error when running "brew doctor":</p>
  
  <pre><code>Error: Your Cellar and TEMP directories are on different volumes.
  OS X won't move relative symlinks across volumes unless the target file already
  exists. Brews known to be affected by this are Git and Narwhal.
  
  You should set the "HOMEBREW_TEMP" environmental variable to a suitable
  directory on the same volume as your Cellar
  </code></pre>
  
  <p>How do I set this variable? And what should I set it to? I can't seem to find anything about this when googling.</p>