Bump nixpkgs
Here we bump nixpkgs and push through quite a few refactorings to fix the fallout. Specifically, several GHC binary derivations that we previously relied upon to bootstrap have been dropped from nixpkgs. We now instead rely on ghc-artefact-nix to provide bootstrap compilers.
We also apply the fix for ghc#19655 since the version of autoconf now supplied by nixpkgs is triggers this issue.
Finally, we now use the Hadrian bootstrap strip to build Hadrian when available.