mixRelease: use C.UTF-8 locale

Avoid the following during mix releases:

```
warning: the VM is running with native name encoding of latin1 which may
cause Elixir to malfunction as it expects utf8. Please ensure your
locale is set to UTF-8 (which can be verified by running "locale" in
your shell)
```

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

@@ -50,6 +50,7 @@ stdenv.mkDerivation (overridable // {
   # some older dependencies still use rebar
   MIX_REBAR = "${rebar}/bin/rebar";
   MIX_REBAR3 = "${rebar3}/bin/rebar3";
+  LC_ALL = "C.UTF-8";
 
   postUnpack = ''
     export HEX_HOME="$TEMPDIR/hex"