Fix argument checks in env plugin

[JSCU-CNI]

This PR fixes a small oversight in the env plugin.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.72%. Comparing base (bdfc048) to head (39ab6dd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/target/plugins/apps/other/env.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #988      +/-   ##
==========================================
- Coverage   77.73%   77.72%   -0.01%     
==========================================
  Files         326      326              
  Lines       28573    28575       +2     
==========================================
  Hits        22210    22210              
- Misses       6363     6365       +2     

Flag	Coverage Δ
unittests	77.72% <0.00%> (-0.01%)	⬇️

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