[test] Skip runPackageJsonScript() on windows and macOS

[styfle]

This test is flaky on windows and macOS but its never actually run on those OS's so there is no need to test it there.

https://github.com/vercel/vercel/runs/4478211743?check_suite_focus=true

https://github.com/vercel/vercel/runs/4478211767?check_suite_focus=true

[codecov]

Codecov Report

Merging #7176 (0b5c143) into main (ecb4481) will not change coverage.
The diff coverage is n/a.

❗ Current head 0b5c143 differs from pull request most recent head b4df508. Consider uploading reports for the commit b4df508 to get more accurate results

@@           Coverage Diff           @@
##             main    #7176   +/-   ##
=======================================
  Coverage   49.17%   49.17%           
=======================================
  Files         135      135           
  Lines        5426     5426           
  Branches     1358     1358           
=======================================
  Hits         2668     2668           
  Misses       2745     2745           
  Partials       13       13           

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