Include mkDerivation arguments in the result as a debugging tool

It is a trivial easy-mergeable no-rebuild change, so I am using a pull request for a brief discussion.

I think it would be a useful debugging tool; and every bit helps. It would help me many times — a bit a time. I checked: there is no hash change.

A possible regression is a slight slowdown of evaluation, but note that the same mkDerivation does a lot of work anyway. Optional inclusion would probably make things only slower. It can be made a stdenv adaptor, but then noone will find it.