test(cmd): safer and simpler cmd test env

[Exca-DK]

This change replaces manual environment variable handling with t.Setenv, which simplifies the logic and ensures proper cleanup.

t.Setenv automatically restores the previous value (or removes it if it didn’t exist) using t.Cleanup, making the tests more robust, even in case of a crash.

Also makes my linter happy since its been screaming for some time for usage of os.setenv in tests.

Ref: t.Setenv

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.22%. Comparing base (19cf1c4) to head (f43213b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2565      +/-   ##
==========================================
+ Coverage   75.20%   75.22%   +0.02%     
==========================================
  Files         140      140              
  Lines       16862    16862              
==========================================
+ Hits        12681    12685       +4     
+ Misses       3353     3350       -3     
+ Partials      828      827       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[weiihann]

LGTM, thanks for the PR!