fix(pgsodium): use pkgs.pkgsLinux for psql_15/psql_17 to match postgresqlWithExtension

The pgsodium test uses pkgs.pkgsLinux.buildEnv to build Linux packages,
but psql_15 and psql_17 were referencing pkgs.system which could be
Darwin. This caused mixed Darwin/Linux binaries in the test environment,
leading to "cannot execute binary file" errors on Darwin when trying
to run pg_regress. Now consistently uses pkgs.pkgsLinux for all package
references.

Author: yvan-sraka

nix/ext/tests/pgsodium.nix

@@ -48,8 +48,12 @@ let
       echo 0000000000000000000000000000000000000000000000000000000000000000
     ''
   );
-  psql_15 = postgresqlWithExtension self.packages.${pkgs.system}.postgresql_15;
-  psql_17 = postgresqlWithExtension self.packages.${pkgs.system}.postgresql_17;
+  psql_15 =
+    postgresqlWithExtension
+      self.packages.${pkgs.pkgsLinux.stdenv.hostPlatform.system}.postgresql_15;
+  psql_17 =
+    postgresqlWithExtension
+      self.packages.${pkgs.pkgsLinux.stdenv.hostPlatform.system}.postgresql_17;
 in
 pkgs.testers.runNixOSTest {
   name = pname;