utils: Homebrew.system respect stderr

[apainintheneck]

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Small fix for one thing brought up in #16134 (comment).

This caused some JSON output to break recently which is not ideal.

The change here is to check if the output should be diverted to stderr based on the options passed to system. If :out => :err is passed, then we should send any verbose output to stderr as well.

$ brew ruby -e 'Homebrew.system("ls", :out => :err)' 2>| wc -c
     105
$ brew ruby -e 'Homebrew.system("ls")' | wc -c
     105

[MikeMcQuaid]

Makes sense, thanks @apainintheneck!