use rustix process functions

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.21%. Comparing base (a995b90) to head (b9e9ea4).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/pgrep/src/process.rs	0.00%	1 Missing ⚠️
src/uu/pidof/src/pidof.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #701   +/-   ##
=======================================
  Coverage   19.21%   19.21%           
=======================================
  Files          68       68           
  Lines       10150    10150           
  Branches      546      546           
=======================================
  Hits         1950     1950           
  Misses       8200     8200           

Flag	Coverage Δ
macos_latest	6.74% <0.00%> (ø)
ubuntu_latest	19.56% <50.00%> (ø)
windows_latest	0.02% <ø> (ø)

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