tests.nixpkgs-check-by-name: Fix non-reproducible test failures

This was an oversight in https://github.com/NixOS/nixpkgs/pull/254435

pkgs/test/nixpkgs-check-by-name/default.nix

@@ -26,10 +26,13 @@ let
         export NIX_STATE_DIR=$TEST_ROOT/var/nix
         export NIX_STORE_DIR=$TEST_ROOT/store
 
-        # cargo tests run in parallel by default, which would then run into
-        # https://github.com/NixOS/nix/issues/2706 unless the store is initialised first
+        # Ensure that even if tests run in parallel, we don't get an error
+        # We'd run into https://github.com/NixOS/nix/issues/2706 unless the store is initialised first
         nix-store --init
       '';
+      # The tests use the shared environment variables,
+      # so we cannot run them in parallel
+      dontUseCargoParallelTests = true;
       postCheck = ''
         cargo fmt --check
         cargo clippy -- -D warnings