python3Packages.pytest-xdist: change pytestFlagsArray in preCheck hook

We don't want to depend on bash eval in pytestFlagsArray to be able to
remove it, thus move it to bash code in preCheck.

pkgs/development/python-modules/pytest-xdist/default.nix

@@ -45,10 +45,10 @@ buildPythonPackage rec {
     setproctitle = [ setproctitle ];
   };
 
-  pytestFlagsArray = [
-    # pytest can already use xdist at this point
-    "--numprocesses=$NIX_BUILD_CORES"
-  ];
+  # pytest can already use xdist at this point
+  preCheck = ''
+    appendToVar pytestFlagsArray "--numprocesses=$NIX_BUILD_CORES"
+  '';
 
   # access file system
   disabledTests = [