buildMix: use C.UTF-8 locale

Avoid the following during mix builds:

warning: the VM is running with native name encoding of latin1 which may
cause Elixir to malfunction as it expects utf8.

pkgs/development/beam-modules/build-mix.nix

@@ -28,6 +28,7 @@ let
     MIX_ENV = mixEnv;
     MIX_DEBUG = if enableDebugInfo then 1 else 0;
     HEX_OFFLINE = 1;
+    LC_ALL = "C.UTF-8";
 
     # add to ERL_LIBS so other modules can find at runtime.
     # http://erlang.org/doc/man/code.html#code-path