Add MIX_ARCHIVES env variable

Author: José Valim

lib/mix/lib/mix/local.ex

@@ -3,9 +3,18 @@ defmodule Mix.Local do
 
   @doc """
   The path for local archives.
+
+  It checks the `MIX_ARCHIVES` variable or it uses the
+  "archives" directory inside `Mix.Utils.mix_home/0`.
+
+  Since archives are specific to Elixir versions, it is
+  expected from build tools to swap the `MIX_ARCHIVES`
+  variable to different locations based on a particular
+  Elixir installation.
   """
   def archives_path do
-    Path.join Mix.Utils.mix_home, "archives"
+    System.get_env("MIX_ARCHIVES") ||
+      Path.join(Mix.Utils.mix_home, "archives")
   end
 
   @doc """

lib/mix/lib/mix/utils.ex

@@ -9,11 +9,12 @@ defmodule Mix.Utils do
   It defaults to `~/.mix` unless the `MIX_HOME`
   environment variable is set.
 
-  Mix stores archives and configurations in the `MIX_HOME`
-  directory. Since archives and configurations may be
-  specific to an Elixir version, it is expected from build
-  tools to swap the `MIX_HOME` to different locations
-  based on particular Elixir installations.
+  Developers should only store entries in the
+  `MIX_HOME` directory which are guaranteed to
+  work accross multiple Elixir versions, as it is
+  not recommended to swap the `MIX_HOME` directory
+  as configuration and other important data may be
+  stored there.
   """
   def mix_home do
     System.get_env("MIX_HOME") || Path.expand("~/.mix")