Gate MacOS calls behind OS.mac? conditionals

[Rylan12]

See Homebrew/brew#16224

This PR makes it so that all references to the MacOS module are gates behind OS.mac? calls to allow the MacOS module to be deprecated on Linux.

My only outstanding question is whether this reference needs to be changed:

homebrew-test-bot/spec/stub/os.rb

Lines 3 to 9 in 9da1d1f

	module MacOS
	module_function
	def version
	:catalina
	end
	end

This is in the test directory so it shouldn't affect functionality, only CI in this repo.

[Bo98]

My only outstanding question is whether this reference needs to be changed:

It doesn't need to be changed, no. You could make it conditional to Darwin to force it to error on Linux if encountered, but that doesn't matter much.

The design of the rspecs were aimed to test test-bot without a brew install, so we have stubbed versions of os.rb.

The rspecs in this repo is very poor anyway atm and hardly do anything. It certainly won't be hitting this sudo purge env line yet.

[MikeMcQuaid]

Thanks @Rylan12 for the PR and @Bo98 for the review and merge!