circleci: Simplify Hadrian build
circleci: Simplify Hadrian build

This uses the build.sh script included in the Hadrian tree, ensuring that we
will build Cabal from git if necessary.

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