Always unpack the source tarball, not the bottle

Library/Contributions/cmds/brew-unpack.rb

@@ -23,6 +23,9 @@ module ScriptDataReader
   end
 end
 
+# otherwise we may unpack bottles
+ENV['HOMEBREW_BUILD_FROM_SOURCE'] = '1'
+
 # Need to tweak the Formula class slightly so that patching is option and `DATA`
 # patches work correctly.
 class Formula