Set up cachix as emacs-overlay binary cache

modules/cachix.nix

@@ -0,0 +1,13 @@
+# WARN: this file will get overwritten by $ cachix use <name>
+{ pkgs, lib, ... }:
+
+let
+  folder = ../cachix;
+  toImport = name: value: folder + ("/" + name);
+  filterCaches = key: value: value == "regular" && lib.hasSuffix ".nix" key;
+  imports = lib.mapAttrsToList toImport
+    (lib.filterAttrs filterCaches (builtins.readDir folder));
+in {
+  inherit imports;
+  nix.binaryCaches = [ "https://cache.nixos.org/" ];
+}