beam: fix locale errors on darwin

C.UTF-8 is non-standard and generates errors such as:

bash: warning: setlocale: LC_ALL: cannot change locale (C.UTF-8): Bad file descriptor

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

@@ -57,7 +57,8 @@ let
           in
           "[${lib.concatStringsSep "," options}]";
 
-        LC_ALL = "C.UTF-8";
+        LANG = if stdenv.isLinux then "C.UTF-8" else "C";
+        LC_CTYPE = if stdenv.isLinux then "C.UTF-8" else "UTF-8";
 
         # add to ERL_LIBS so other modules can find at runtime.
         # http://erlang.org/doc/man/code.html#code-path

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

@@ -134,7 +134,8 @@ stdenv.mkDerivation (
       in
       "[${lib.concatStringsSep "," options}]";
 
-    LC_ALL = "C.UTF-8";
+    LANG = if stdenv.isLinux then "C.UTF-8" else "C";
+    LC_CTYPE = if stdenv.isLinux then "C.UTF-8" else "UTF-8";
 
     postUnpack =
       ''