Enable binary cache for nix-community

Finally fully enable binary cache for nix-community and emacs-overlay.
No I don't have to build emacs from scratch for every master update :)

This was done by means of cachix, which is a means to upload built derivations
for others to share with. Nix-community has their own, and it makes using
overlays provided by nix-community that much better. This should shave off 20-25
min per system rebuild.

I am not sure if this will work as is after moving this config to new system,
so here are the steps to follow:

1. nix shell nixpkgs#cachix
2. sudo cachix use nix-community
3. stage/commit cachix.nix and cachix/ from /etc/nixos
4. done

Hopefully this will be enough in the future system move. If not, all the best to
future me.

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/"];
+}

cachix/nix-community.nix

@@ -0,0 +1,11 @@
+
+{
+  nix = {
+    binaryCaches = [
+      "https://nix-community.cachix.org"
+    ];
+    binaryCachePublicKeys = [
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+    ];
+  };
+}

flake.nix

@@ -21,6 +21,7 @@
           ./hosts/enterprise/configuration.nix
           { nixpkgs.overlays = [ emacs-overlay.overlay ]; }
           (import ./hosts/enterprise/sound.nix)
+          (import ./cachix.nix)
           home-manager.nixosModules.home-manager
           {
             home-manager.useGlobalPkgs = true;