nixos/tests/incus: pass package to releases config

virtual-machine releases use virtualisation.incus.package to find
where the agent-loader configuration is defined. In practice it's
likely not a problem, but when marking lts v6 on 25.11 as vulnerable
it exposed the mismatch. For correctness we should ensure the VM
tests are using the agent loader from the relevant incus package.

nixos/tests/incus/incus-tests-module.nix

@@ -1,10 +1,12 @@
 {
+  config,
   lib,
   pkgs,
   ...
 }:
 let
   jsonFormat = pkgs.formats.json { };
+  cfg = config.tests.incus;
 in
 {
   options.tests.incus = {
@@ -74,7 +76,11 @@ in
             config =
               let
                 releases = import ../../release.nix {
-                  configuration = config.nixosConfig;
+                  configuration = lib.recursiveUpdate config.nixosConfig {
+                    virtualisation.incus = {
+                      inherit (cfg) package;
+                    };
+                  };
                 };
 
                 images = {