docs: Allow BUILDDIR to be set from environment

[victorlin]

Mirrors nextstrain/docs.nextstrain.org#232

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.24%. Comparing base (3f588da) to head (da76c33).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1650    +/-   ##
========================================
  Coverage   72.24%   72.24%            
========================================
  Files          79       79            
  Lines        8268     8268            
  Branches     2010     1693   -317     
========================================
  Hits         5973     5973            
- Misses       1997     2009    +12     
+ Partials      298      286    -12     

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