make it so servo doesn't do binfmt emulation, nor fetch cache from desko

hosts/by-name/desko/default.nix

@@ -4,7 +4,7 @@
     ./fs.nix
   ];
 
-  sane.roles.build-machine = true;
+  sane.roles.build-machine.enable = true;
   sane.roles.client = true;
   sane.services.wg-home.enable = true;
   sane.services.wg-home.ip = config.sane.hosts.by-name."desko".wg-home.ip;

hosts/by-name/servo/default.nix

@@ -15,7 +15,8 @@
     signaldctl.enableFor.user.colin = true;
   };
 
-  sane.roles.build-machine = true;
+  sane.roles.build-machine.enable = true;
+  sane.roles.build-machine.emulation = false;
   sane.zsh.showDeadlines = false;  # ~/knowledge doesn't always exist
   sane.services.dyn-dns.enable = true;
   sane.services.wg-home.enable = true;

hosts/modules/nixcache.nix

@@ -28,8 +28,9 @@ in
     sane.nixcache.substituters = mkOption {
       type = types.listOf types.string;
       default =
+        # TODO: make these blacklisted entries injectable
         (lib.optional (hostName != "servo") "https://nixcache.uninsane.org")
-        ++ (lib.optional (hostName != "desko") "http://desko:5000")
+        ++ (lib.optional (hostName != "servo" && hostName != "desko") "http://desko:5000")
         ++ [
           "https://nix-community.cachix.org"
           "https://cache.nixos.org/"

hosts/modules/roles/build-machine.nix

@@ -3,11 +3,18 @@
 let
   inherit (lib) mkIf mkMerge mkOption types;
   inherit (config.programs.ccache) cacheDir;
+  cfg = config.sane.roles.build-machine;
 in
 {
-  options.sane.roles.build-machine = mkOption {
-    type = types.bool;
-    default = false;
+  options.sane.roles.build-machine = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+    };
+    emulation = mkOption {
+      type = types.bool;
+      default = true;
+    };
   };
 
   config = mkMerge [
@@ -30,13 +37,13 @@ in
       #   })
       # ];
     }
-    (mkIf config.sane.roles.build-machine {
+    (mkIf cfg.enable {
       # serve packages to other machines that ask for them
       sane.services.nixserve.enable = true;
 
       # enable cross compilation
       # TODO: do this via stdenv injection, linking into /run/binfmt the stuff in <nixpkgs:nixos/modules/system/boot/binfmt.nix>
-      boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
+      boot.binfmt.emulatedSystems = lib.optional cfg.emulation [ "aarch64-linux" ];
       # corresponds to env var: NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1
       # nixpkgs.config.allowUnsupportedSystem = true;