fix: testing for flag files on windows

[piksel]

Since testing for file names that contain a : (and not being a legitimate volume letter path, like c:) causes a fatal error on windows, this skips checking such strings and always considers them being literal flag values.

Fixes #1245

[codecov]

Codecov Report

Merging #1249 (93b0897) into main (a5c60a9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
+ Coverage   63.00%   63.02%   +0.02%     
==========================================
  Files          23       23              
  Lines        1500     1501       +1     
==========================================
+ Hits          945      946       +1     
  Misses        471      471              
  Partials       84       84              

Impacted Files	Coverage Δ
internal/flags/flags.go	88.85% <100.00%> (+0.03%)	⬆️

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