CHEF-2185 Fix the client trying to modify frozen string in enforce_path_sanity

Author: Nuo Yan

chef/lib/chef/client.rb

@@ -317,8 +317,10 @@ def enforce_path_sanity(env=ENV)
         existing_paths = env["PATH"].split(':')
         SANE_PATHS.each do |sane_path|
           unless existing_paths.include?(sane_path)
-            env["PATH"] << ':' unless env["PATH"].empty?
-            env["PATH"] << sane_path
+            env_path = env["PATH"].dup
+            env_path << ':' unless env["PATH"].empty?
+            env_path << sane_path
+            env["PATH"] = env_path
           end
         end
       end