nixosTests.nginx-etag: migrate to runTest

Part of #386873

nixos/tests/all-tests.nix

···
   nghttpx = handleTest ./nghttpx.nix {};
   nginx = runTest ./nginx.nix;
   nginx-auth = runTest ./nginx-auth.nix;
-  nginx-etag = handleTest ./nginx-etag.nix {};
+  nginx-etag = runTest ./nginx-etag.nix;
   nginx-etag-compression = handleTest ./nginx-etag-compression.nix {};
   nginx-globalredirect = handleTest ./nginx-globalredirect.nix {};
   nginx-http3 = handleTest ./nginx-http3.nix {};

nixos/tests/nginx-etag.nix

···
-import ./make-test-python.nix {
+{ ... }:
+{
   name = "nginx-etag";
 
   nodes = {
···
   testScript =
     { nodes, ... }:
     let
-      inherit (nodes.server.config.system.build) toplevel;
+      inherit (nodes.server.system.build) toplevel;
       newSystem = "${toplevel}/specialisation/pass-checks";
     in
     ''