title: Undefined cran_mirror when building conda packages
body: <p>I am trying to build an R package in conda using skeleton. After preparing the environment with the required packages, I execute the following commands</p>
  
  <pre><code>$ conda skeleton cran arules
  $ conda build r-arules
  </code></pre>
  
  <p>The recipe files are generated and everything seems OK, but the build command results into the following error:</p>
  
  <pre><code>Adding in variants from internal_defaults
  INFO:conda_build.variants:Adding in variants from internal_defaults
  Attempting to finalize metadata for r-arules
  INFO:conda_build.metadata:Attempting to finalize metadata for r-arules
  Undefined Jinja2 variables remain (['cran_mirror', 'cran_mirror']). 
  Please enable source downloading and try again.
  </code></pre>
  
  <p>I've been reading the documentation and searching the Web for a solution to this problem, without success, and I do not know what else to do. ¿Any ideas?</p>
  
  <p><strong>Notes</strong>: I've started following this tutorial: <a href="http://ihrke.github.io/conda.html" rel="nofollow noreferrer">http://ihrke.github.io/conda.html</a> but afterwards I have also checked the official <a href="https://conda.io/docs/user-guide/tasks/build-packages/index.html" rel="nofollow noreferrer">conda documentation on building packages</a> and other related docs. I have updated conda (= 4.5.2), as well as conda-build ( = 3.10.1)</p>