rubocops: adjust depends_on checks for Linux

[AlternateRT]

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This adjusts the Homebrew/OSDependsOn cop in two ways:

The first change makes it so that declaring depends_on :linux is explicitly required for Linux-only casks as well (like in koreader).

The second change relaxes dependency declarations when using the os stanza; IMO, the current check does not make a lot of sense. Consider a cask that supports both macOS and Linux, and has a URL like this:

url "https://download.example.com/stable#{os}/#{version}/Foo.zip"

Where there is only a differentiator for the macOS artifact with the suffix -darwin.

With the current check, we would either need to declare an empty string for the :linux parameter like so:

os macos: "-darwin", linux: ""

Or replace the os stanza with an on_system_conditional call, which does not have this restriction.

os_suffix = on_system_conditional macos: "-darwin"

[MikeMcQuaid]

Thanks!