Add an xcodebuild helper and deprecate system "xcodebuild"

In the future we can remove the ENV munging branch from Formula#system.

Closes Homebrew/homebrew#27081.

Library/Homebrew/extend/fileutils.rb

@@ -90,4 +90,11 @@ module FileUtils extend self
   def ruby *args
     system RUBY_PATH, *args
   end
+
+  def xcodebuild *args
+    removed = ENV.remove_cc_etc
+    system "xcodebuild", *args
+  ensure
+    ENV.update(removed)
+  end
 end

Library/Homebrew/formula.rb

@@ -565,6 +565,7 @@ class Formula
     ohai "#{cmd} #{pretty_args*' '}".strip
 
     if cmd.to_s.start_with? "xcodebuild"
+      opoo %{system "xcodebuild" is deprecated, use the xcodebuild method instead}
       removed_ENV_variables.update(ENV.remove_cc_etc)
     end