test(flags): ensure temp files are cleaned up

[piksel]

Before, the tests actually left temporary files on windows due to floating file handles. This fix will fix warnings in flags and flags_test and ensures that the temporary files are removed (or it will fail the tests).

[codecov]

Codecov Report

Patch coverage: 52.94% and project coverage change: -0.12% ⚠️

Comparison is base (32204a7) 67.70% compared to head (abeed88) 67.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1727      +/-   ##
==========================================
- Coverage   67.70%   67.59%   -0.12%     
==========================================
  Files          26       26              
  Lines        2378     2379       +1     
==========================================
- Hits         1610     1608       -2     
- Misses        670      672       +2     
- Partials       98       99       +1     

Files Changed	Coverage Δ
internal/flags/flags.go	85.17% <52.94%> (-0.54%)	⬇️

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[piksel]

Unfortunately, this PR lowers the test coverage, but not really. The same paths were just as untested before (they just logged the error and panicked or ignored the them). Additionally, the untested lines are pretty hard to simulate (I/O errors), or could never happen unless the actual code is incorrect (a flag that has secret file lookup enabled, but doesn't exist in the flags).