Use `add-source` instead of linking cabal.sandbox.config
Use `add-source` instead of linking cabal.sandbox.config

As suggested by mpickering

sdiehl/ghc,sdiehl/ghc,izgzhen/hadrian,snowleopard/hadrian,snowleopard/shaking-up-ghc,bgamari/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc