More CI updates

[EmmEff]

• Bump golangci-lint to v2.1.x
• Resolve issues discovered by same (incl. implementing build subcommand argument validation unit tests)

[codecov]

Codecov Report

Attention: Patch coverage is 61.19403% with 26 lines in your changes missing coverage. Please review.

Project coverage is 48.66%. Comparing base (6d99a79) to head (ff001e0).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/app/buildclient/pgpSigner.go	0.00%	10 Missing ⚠️
internal/app/buildclient/build.go	78.12%	7 Missing ⚠️
internal/app/buildclient/artifact.go	0.00%	2 Missing ⚠️
internal/app/buildclient/client.go	77.77%	2 Missing ⚠️
internal/app/buildclient/files.go	71.42%	2 Missing ⚠️
client/build.go	0.00%	1 Missing ⚠️
client/output.go	80.00%	1 Missing ⚠️
internal/app/buildclient/sign.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
+ Coverage   47.48%   48.66%   +1.17%     
==========================================
  Files          19       19              
  Lines        1312     1344      +32     
==========================================
+ Hits          623      654      +31     
- Misses        621      622       +1     
  Partials       68       68              

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