Check if pytestFlagsArray contains "-n XXX" when passing in pytest-xdist to checkInputs

[SuperSandro2000]

Got the idea from NixOS/nixpkgs#114473 (comment).

  checkInputs = [ pytest-xdist ];

  pytestFlagsArray = [
  ...
  "-n auto"
  ...
  ];

or

  pytestFlagsArray = [
  ...
  "-n 2"
  ...
  ];

[piegamesde]

This might even work with some command line parsers, although it is uncommon. But arguments like "-n=auto" or even "-nauto" (it hurts less in the "-n2" variant) may work as well.