fixing a test to also work in Windows

used to do this:
'MIX_ENV' is not recognized as an internal or external command,
operable program or batch file.
Tested to work on both Windows and Linux.
However the following is seen on console:
Mix requires the compile path "ebin" to be inside a directory with the
 same name as the application name :no_mixfile,
got "s:/workspace.elixir.2013/elixir/lib/mix/tmp/Mix.CLITest/test env/ebin"

quick test with `make test_mix`

Author: AtKaaZ

lib/mix/test/mix/cli_test.exs

@@ -5,7 +5,13 @@ defmodule Mix.CLITest do
 
   test "env" do
     in_fixture "only_mixfile", fn ->
-      env = System.cmd %b(MIX_ENV=prod #{elixir_executable} #{mix_executable} run "IO.puts Mix.env")
+      if match? { :win32, _ }, :os.type do
+        temp_env = "set MIX_ENV=prod &"
+      else # ie. linux
+        temp_env = "MIX_ENV=prod"
+      end
+
+      env = System.cmd %b(#{temp_env} #{elixir_executable} #{mix_executable} run -e "IO.puts Mix.env")
       assert env =~ "prod"
     end
   end