Add BLACKBOX_ENV env variable

[Baptouuuu]

This is a variable used in https://github.com/Innmind/github-workflows/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.10%. Comparing base (d021214) to head (2fedc9a).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #13   +/-   ##
==========================================
  Coverage      96.10%   96.10%           
  Complexity       267      267           
==========================================
  Files             34       34           
  Lines           2617     2618    +1     
==========================================
+ Hits            2515     2516    +1     
  Misses           102      102           

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