feat(rules): enhance subprocess variable checks

[oittaa]

fixes #1174

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 76.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.00%. Comparing base (1216c9b) to head (b4d3ab4).
⚠️ Report is 152 commits behind head on master.

Files with missing lines	Patch %	Lines
rules/subproc.go	76.00%	4 Missing and 2 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1453      +/-   ##
==========================================
- Coverage   68.49%   65.00%   -3.50%     
==========================================
  Files          75       80       +5     
  Lines        4384     4840     +456     
==========================================
+ Hits         3003     3146     +143     
- Misses       1233     1546     +313     
  Partials      148      148              

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