Pass through ZOO_ENGINE_PR

[iterion]

Not really documenting this as it's only for internal use atm. But, it'd be really helpful to have this available.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.88%. Comparing base (fb34e29) to head (a6e50f7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/context.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1656      +/-   ##
==========================================
- Coverage   63.95%   63.88%   -0.07%     
==========================================
  Files          44       44              
  Lines        8117     8118       +1     
==========================================
- Hits         5191     5186       -5     
- Misses       2926     2932       +6     

Flag	Coverage Δ
unittests	63.88% <0.00%> (-0.07%)	⬇️

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