Don't run GHC -M concurrently with ghc-pkg.
Don't run GHC -M concurrently with ghc-pkg.

See #205.

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