nixosTests.nixos-rebuild-specialisations: Use modular entrypoint

Modernize it. This allows the test to be extended, and pkgs to be
reused (later) to speed up evaluations a bit.
I believe this also makes it run on darwin hosts, but my linux-builder's
disk is too small to fit the massive closure of this test.

(cherry picked from commit 1396a03bee18a0993a4f3e97fda8938ff61c2918)

nixos/tests/nixos-rebuild-specialisations.nix

@@ -1,6 +1,10 @@
-import ./make-test-python.nix ({ pkgs, ... }: {
+{ hostPkgs, ... }: {
   name = "nixos-rebuild-specialisations";
 
+  # TODO: remove overlay from  nixos/modules/profiles/installation-device.nix
+  #        make it a _small package instead, then remove pkgsReadOnly = false;.
+  node.pkgsReadOnly = false;
+
   nodes = {
     machine = { lib, pkgs, ... }: {
       imports = [
@@ -32,7 +36,7 @@ import ./make-test-python.nix ({ pkgs, ... }: {
 
   testScript =
     let
-      configFile = pkgs.writeText "configuration.nix" ''
+      configFile = hostPkgs.writeText "configuration.nix" ''
         { lib, pkgs, ... }: {
           imports = [
             ./hardware-configuration.nix
@@ -119,4 +123,4 @@ import ./make-test-python.nix ({ pkgs, ... }: {
           machine.fail("nixos-rebuild boot --specialisation foo")
           machine.fail("nixos-rebuild boot -c foo")
     '';
-})
+}