tests, example: avoid lengthy documentation build

This options manual rebuild takes 30-60s and is triggered by the extra
NixOS options defined by nix-bitcoin.

examples/qemu-vm/minimal-vm.nix

@@ -29,6 +29,9 @@ rec {
       # For faster startup in offline VMs
       services.clightning.extraConfig = "disable-dns";
 
+      # Avoid lengthy build of the nixos manual
+      documentation.nixos.enable = false;
+
       nixpkgs.pkgs = pkgs;
       services.getty.autologinUser = "root";
       nix.nixPath = [ "nixpkgs=${nixpkgs}" ];

test/lib/make-test.nix

@@ -94,6 +94,9 @@ name: testConfig:
     virtualisation.graphics = false;
     services.getty.autologinUser = "root";
 
+    # Avoid lengthy build of the nixos manual
+    documentation.nixos.enable = false;
+
     # Provide a shortcut for instant poweroff from within the machine
     environment.systemPackages = with pkgs; [
       (lowPrio (writeScriptBin "q" ''