guile-cairo: use checkInputs

pkgs/development/guile-modules/guile-cairo/default.nix

···
 
   nativeBuildInputs = [ pkgconfig ];
 
-  buildInputs = [ guile cairo expat ]
-    ++ stdenv.lib.optional doCheck guile-lib;
+  buildInputs = [ guile cairo expat ];
+  checkInputs = [ guile-lib ];
 
   doCheck = true;