nixops_unstable: Fix tests attribute

The .overrideAttrs part is ok now, but a "passthru' workaround is
necessary now. See https://github.com/NixOS/nixpkgs/pull/247520

nixos/tests/nixops/default.nix

···
     # inherit testsForPackage;
   };
 
-  testsForPackage = lib.makeOverridable (args: lib.recurseIntoAttrs {
+  testsForPackage = args: lib.recurseIntoAttrs {
     legacyNetwork = testLegacyNetwork args;
-  });
+    passthru.override = args': testsForPackage (args // args');
+  };
 
-  testLegacyNetwork = { nixopsPkg }: pkgs.nixosTest ({
+  testLegacyNetwork = { nixopsPkg, ... }: pkgs.nixosTest ({
     name = "nixops-legacy-network";
     nodes = {
       deployer = { config, lib, nodes, pkgs, ... }: {
···
           chmod 0400 ~/.ssh/id_ed25519
         '';
         serverNetworkJSON = pkgs.writeText "server-network.json"
-          (builtins.toJSON nodes.server.config.system.build.networkConfig);
+          (builtins.toJSON nodes.server.system.build.networkConfig);
       in
       ''
         import shlex

pkgs/applications/networking/cluster/nixops/default.nix

···
     }
   ).python;
 
-  pkg = interpreter.pkgs.nixops.withPlugins(ps: [
+  pkg = (interpreter.pkgs.nixops.withPlugins(ps: [
     ps.nixops-aws
     ps.nixops-digitalocean
     ps.nixops-encrypted-links
···
     ps.nixopsvbox
     ps.nixops-virtd
     ps.nixops-hetznercloud
-  ]) // rec {
-    # Workaround for https://github.com/NixOS/nixpkgs/issues/119407
-    # TODO after #1199407: Use .overrideAttrs(pkg: old: { passthru.tests = .....; })
-    tests = nixosTests.nixops.unstable.override { nixopsPkg = pkg; };
-    # Not strictly necessary, but probably expected somewhere; part of the workaround:
-    passthru.tests = tests;
-  };
+  ])).overrideAttrs (finalAttrs: prevAttrs: {
+    passthru = prevAttrs.passthru or {} // {
+      tests = prevAttrs.passthru.tests or {} //
+        nixosTests.nixops.unstable.passthru.override { nixopsPkg = pkg; };
+    };
+  });
 in pkg