Allow setting `sourceRoot` and `setSourceRoot` in the arguments to
`haskellPackages.mkDerivation`. These arguments are passed directly to
`stdenv.mkDerivation`.
It may be worth reading over `pkgs/stdenv/generic/setup.sh` and checking
if there's other unsupported variables which may be useful. It may also
be worth adding an argument like `extraAttrs` to be passed to
`stdenv.mkDerivation` directly, as an escape hatch.
ff9e35236c