Adapt to nixpkgs' nativeCheckInputs change

nixpkgs commit f39abbc3500451fc00c80fed71b0f6f7bba8b6a4, first released in
23.05, requires that everyone move their checkInputs to nativeCheckInputs.
To build both before and after this change, we detect whether we're before
or after this change by reading the manual & behave appropriately.  :/