Make `hsc2hs` less magic in haskell.nix

[hamishmack]

Currently we include the GHC bundled hsc2hs automatically for every build. It would be nicer if we respected the build-tool-depends selected version in the cabal plan.

haskell.nix/builder/comp-builder.nix

Line 175 in cebbb6d

"--with-hsc2hs=${ghc.targetPrefix}hsc2hs"

haskell.nix/builder/comp-builder.nix

Lines 244 to 246 in cebbb6d

	# We always exclude hsc2hs from build-tools because it is unecessary as it is provided by ghc
	# and hsc2hs from ghc is first in PATH so the one from build-tools is never used.
	&& x.identifier.name or "" != "hsc2hs")