python3Packages.dtw-python: set pytestFlagsArray in preCheck hook

This flag relies on bash eval of pytestFlagsArray, which we'd like to
get rid of. By moving the evaluation of $src into the preCheck hook, we
achieve the same.

pkgs/development/python-modules/dtw-python/default.nix

@@ -39,10 +39,12 @@ buildPythonPackage rec {
   ];
 
   # We need to run tests on real built package: https://github.com/NixOS/nixpkgs/issues/255262
-  preCheck = "cd $out";
-  nativeCheckInputs = [ pytestCheckHook ];
   # tests/ are not included to output package, so we have to set path explicitly
-  pytestFlagsArray = [ "$src/tests" ];
+  preCheck = ''
+    appendToVar pytestFlagsArray "$src/tests"
+    cd $out
+  '';
+  nativeCheckInputs = [ pytestCheckHook ];
 
   pythonImportsCheck = [ "dtw" ];