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)
```
This flag appears to cause issues in 3 out 4 users of mixRelease in
Nixpkgs, plus Mobilizon, which isn't merged yet. It might be safer to
set it to fals for now, as the runtime error messages produced by it are
quite cryptic.
The Elixir LS package in Nixpkgs by default used the latest Elixir
version available to compile and run Elixir LS. The user can build
a custom Elixir LS package with a different Elixir version:
my-custom-elixir-ls = pkgs.elixir-ls.override {
elixir = my-custom-elixir;
};
But by doing so the user changes only the Elixir version used to
run Elixir LS; the Elixir version used to compile Elixir LS doesn't
change. As the result, the custom Elixir LS package uses a different
Elixir version at runtime than the Elixir version it was compiled with.
In order to be able to modify the Elixir version used at build time,
I changed `mixRelease` and `fetchMixDeps` to accept `elixir` and
`hex` as parameters (defaults to the latest Elixir and Hex packages).
