os/mac: fall back to old environment.

Can't see how this could ever happen but let's safe-guard it just in
case (as ilovezfs saw it).

Library/Homebrew/os/mac.rb

@@ -24,7 +24,7 @@ module OS
     # This can be compared to numerics, strings, or symbols
     # using the standard Ruby Comparable methods.
     def full_version
-      @full_version ||= Version.new(ENV["HOMEBREW_MACOS_VERSION"].chomp)
+      @full_version ||= Version.new((ENV["HOMEBREW_MACOS_VERSION"] || ENV["HOMEBREW_OSX_VERSION"]).chomp)
     end
 
     def prerelease?