build-aux/snap/local: cleanup patch and verification helper scripts

[bboozzoo]

Improve the patching and verification scripts used during snap build.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.22%. Comparing base (24a0034) to head (ecdb967).
Report is 83 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14901      +/-   ##
==========================================
+ Coverage   78.20%   78.22%   +0.02%     
==========================================
  Files        1151     1153       +2     
  Lines      151396   152669    +1273     
==========================================
+ Hits       118402   119431    +1029     
- Misses      25662    25864     +202     
- Partials     7332     7374      +42     

Flag	Coverage Δ
unittests	78.22% <ø> (+0.02%)	⬆️

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