fix: nixos test for plv8 extension to use the supabase_admin user

We switched to using the supabase_admin user for our Postgres tests instead of the default postgres user.
This change updates the authentication methods and user management in the NixOS test configurations for the plv8 extension.

Author: jfroche

nix/ext/tests/plv8.nix

@@ -60,6 +60,20 @@ self.inputs.nixpkgs.lib.nixos.runTest {
       services.postgresql = {
         enable = true;
         package = postgresqlWithExtension self.packages.${pkgs.system}.postgresql_15;
+        authentication = ''
+          local all postgres peer map=postgres
+          local all all peer map=root
+        '';
+        identMap = ''
+          root root supabase_admin
+          postgres postgres postgres
+        '';
+        ensureUsers = [
+          {
+            name = "supabase_admin";
+            ensureClauses.superuser = true;
+          }
+        ];
       };
     };
   testScript =