Add missing SystemCommand::Mixin to fix CI

[dduugg]

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Should resolve https://github.com/Homebrew/brew/actions/runs/7733313670/job/21085133518

[Bo98]

Also:

Error: undefined method `system_command' for Homebrew:Module
Do not report this issue until you've run `brew update` and tried again.
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/update-sponsors.rb:64:in `update_sponsors'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:86:in `<main>'
Error: undefined method `system_command' for Homebrew:Module
Do not report this issue until you've run `brew update` and tried again.
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/update-maintainers.rb:57:in `update_maintainers'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:86:in `<main>'

[dduugg]

Also:

Error: undefined method `system_command' for Homebrew:Module
Do not report this issue until you've run `brew update` and tried again.
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/update-sponsors.rb:64:in `update_sponsors'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:86:in `<main>'
Error: undefined method `system_command' for Homebrew:Module
Do not report this issue until you've run `brew update` and tried again.
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/update-maintainers.rb:57:in `update_maintainers'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:86:in `<main>'

Yeah, there might be a long tail of these, as tests are presumably run in random orders. Since commands all live in a single namespace, sorbet can't detect which need to have the mixin, and load order determines what's been mixed in at runtime. Anyway, fix incoming.