Synchronize Nix configuration with other Witchcraft projects

These changes had already been applied to the other
projects within the Witchcraft suite, and add support
for working with the project on apple silicon.

.envrc

@@ -1 +1,4 @@
-eval "$(lorri direnv)"
+use nix
+
+export MIX_HOME=$(pwd)/.mix
+export PATH=$PATH:$(pwd)/.mix/escripts

.gitignore

@@ -7,3 +7,4 @@ erl_crash.dump
 *.beam
 .DS_Store
 mix.lock
+.mix

shell.nix

@@ -1,45 +1,21 @@
- let
-  sources  = import ./nix/sources.nix;
-  commands = import ./nix/commands.nix;
-
-  nixos    = import sources.nixpkgs  {};
-  darwin   = import sources.darwin   {};
-  unstable = import sources.unstable {};
-
-  pkgs  = if darwin.stdenv.isDarwin then darwin else nixos;
-  tasks = commands {
-    inherit pkgs;
-    inherit unstable;
-  };
-
-  deps = {
-    common =
-      [  pkgs.niv
-      ];
-
-    elixir =
-      [ unstable.elixir
-      ];
-
-    platform =
-      if pkgs.stdenv.isDarwin then
-        [ unstable.darwin.apple_sdk.frameworks.CoreServices
-          unstable.darwin.apple_sdk.frameworks.Foundation
-        ]
-      else if pkgs.stdenv.isLinux then
-        [ pkgs.inotify-tools
-        ]
-      else
-        [];
-  };
-in
-
-pkgs.mkShell {
-  name = "Algae";
-  nativeBuildInputs = builtins.concatLists [
-    deps.common 
-    deps.elixir
-    deps.platform
-    tasks
-  ];
+let
+  nixpkgs = import (fetchTarball {
+    # Run `cachix use jechol` to use compiled binary cache.
+    url = "https://github.com/jechol/nixpkgs/archive/21.11-otp24-no-jit.tar.gz";
+    sha256 = "sha256:1lka707hrnkp70vny99m9fmp4a8136vl7addmpfsdvkwb81d1jk9";
+  }) { };
+  platform = if nixpkgs.stdenv.isDarwin then [
+    nixpkgs.darwin.apple_sdk.frameworks.CoreServices
+    nixpkgs.darwin.apple_sdk.frameworks.Foundation
+  ] else if nixpkgs.stdenv.isLinux then
+    [ nixpkgs.inotify-tools ]
+  else
+    [ ];
+in nixpkgs.mkShell {
+  buildInputs = with nixpkgs;
+    [
+      # OTP
+      erlang
+      elixir
+    ] ++ platform;
 }