Hadrian: Allow passing CABFLAGS into build.cabal.sh
Hadrian: Allow passing CABFLAGS into build.cabal.sh

Setting `CABFLAGS=args` will pass the additional arguments to cabal
when it is invoked.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc