docs: Allow BUILDDIR to be set from environment

[victorlin]

Mirrors https://github.com/nextstrain/docs.nextstrain.org/pull/232

Checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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