Allow MUJOCO_DIR to be specified as cmake variable

[rhaschke]

So far, MUJOCO_DIR had to be an environment variable. Passing it as a cmake variable (-DMUJOCO_DIR=...) didn't work.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.15%. Comparing base (f640ba0) to head (ed29627).
Report is 1 commits behind head on noetic-devel.

Additional details and impacted files

@@               Coverage Diff                @@
##           noetic-devel      #41      +/-   ##
================================================
+ Coverage         59.31%   60.15%   +0.85%     
================================================
  Files                19       19              
  Lines              2860     2820      -40     
  Branches            334      333       -1     
================================================
  Hits               1696     1696              
+ Misses             1164     1124      -40     

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