cachix: default binary cache is nixos.org, no need

to specify it explicitly (results in double entry in nix.conf)

Also force our nix.extraOptions because digga tries real hard to put
their binary caches there:
0595ae70cd/modules/nix-config.nix (L19-L23)

modules/core/nix.nix

@@ -21,7 +21,7 @@
       system-features = ["nixos-test" "benchmark" "big-parallel" "kvm"];
     };
     # Generally useful nix option defaults
-    extraOptions = ''
+    extraOptions = lib.mkForce ''
       min-free = 536870912
       keep-outputs = true
       keep-derivations = true

profiles/cachix/default.nix

@@ -1,13 +0,0 @@
-{
-  pkgs,
-  lib,
-  ...
-}: let
-  folder = ./.;
-  toImport = name: value: folder + ("/" + name);
-  filterCaches = key: value: value == "regular" && lib.hasSuffix ".nix" key && key != "default.nix";
-  imports = lib.mapAttrsToList toImport (lib.filterAttrs filterCaches (builtins.readDir folder));
-in {
-  inherit imports;
-  nix.settings.substituters = ["https://cache.nixos.org/"];
-}

profiles/full-install/default.nix

@@ -7,8 +7,6 @@
 }: let
   inherit (lib) fileContents;
 in {
-  imports = [../cachix];
-
   config = {
     pub-solar.audio.mopidy.enable = true;
     pub-solar.audio.bluetooth.enable = true;