[lit] Reverse path list when updating environment vars.

Bug pointed out by EricWF.  This would construct a path where
items would be added in the wrong order, potentially leading
to using the wrong tools for testing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313765 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed