brew-unpack: define fails_with_llvm? as false

brew-unpack was failing for formulae that satisfy fails_with_llvm? as
handle_llvm_failure() requires our ENV extension to get ENV.compiler.

Rather than requiring extend/ENV, just define fails_with_llvm? as false
since we don't really care about that when just unpacking a formula.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Contributions/examples/brew-unpack.rb

@@ -39,6 +39,10 @@ class Formula
       do_patch
     end
   end
+
+  # handle_llvm_failure() requires extend/ENV, so let's never fail
+  # with llvm since we don't particularly care in this context.
+  def fails_with_llvm?; false; end
 end
 
 module Homebrew extend self