title: How should I interpolate environment variables in Shake file patterns?
body: <p>In my Makefiles, I prefer having the output directory defined by a environment variable rather than hard-coded (with some reasonable default value if its unset). For example, a Make rule would look like</p>
  
  <pre><code>$(OUTPUT_DIR)/some_file: deps
       #build commands
  </code></pre>
  
  <p>I have yet to figure out how to achieve a similar goal in Shake. I like using <code>getEnvWithDefault</code> to grab the value of the environment variable or a reasonable default, but no amount of bashing it with binds or lambdas have allowed me to combine it with <code>(*&gt;)</code>.</p>
  
  <p>How might it be possible to interpolate an environment variable in a FilePattern for use with <code>(*&gt;)</code>?</p>