requirement: prepend rather than append PATH.

Otherwise this ends up behind e.g. `/usr/bin` so is pretty useless.

Library/Homebrew/requirement.rb

@@ -95,7 +95,7 @@ class Requirement
     parent = satisfied_result_parent
     return unless parent
     return if PATH.new(ENV["PATH"]).include?(parent.to_s)
-    ENV.append_path("PATH", parent)
+    ENV.prepend_path("PATH", parent)
   end
 
   def env

Library/Homebrew/test/requirement_spec.rb

@@ -138,7 +138,7 @@ describe Requirement do
       end
 
       it "infers path from #satisfy result" do
-        expect(ENV).to receive(:append_path).with("PATH", Pathname.new("/foo/bar"))
+        expect(ENV).to receive(:prepend_path).with("PATH", Pathname.new("/foo/bar"))
         subject.satisfied?
         subject.modify_build_environment
       end