pgrep: optimize length detection

[Franklin-Qi]

When calculating the length, the ^$ character needs to be removed if the -x parameter is present.

Closes: #635

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 18.76%. Comparing base (6ac0e01) to head (961f167).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/pgrep/src/process_matcher.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #636      +/-   ##
==========================================
- Coverage   18.77%   18.76%   -0.02%     
==========================================
  Files          66       66              
  Lines        9752     9758       +6     
  Branches      516      518       +2     
==========================================
  Hits         1831     1831              
- Misses       7921     7927       +6     

Flag	Coverage Δ
macos_latest	6.90% <0.00%> (-0.02%)	⬇️
ubuntu_latest	19.12% <0.00%> (-0.02%)	⬇️
windows_latest	0.02% <0.00%> (-0.01%)	⬇️

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[Krysztal112233]

could you please add some unittest to test your modification and make sure no regression if possible?