formula_assertions: Fix type of cmd param in shell_output

[issyl0]

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• This can be either a String or a Pathname, per the part of the noseyparker test that failed (in a different part of the test, the command is passed as a string).

  ==> Testing noseyparker
  ==> /opt/homebrew/Cellar/noseyparker/0.18.1/bin/noseyparker -V
  Error: noseyparker: failed
  An exception occurred within a child process:
    TypeError: Parameter 'cmd': Expected type String, got type Pathname with value #<Pathname:/opt/homebrew/Ce...ps://github.com/Homebrew/brew>
  Caller: /opt/homebrew/Library/Taps/homebrew/homebrew-core/Formula/n/noseyparker.rb:35

[samford]

Thanks, @issyl0!