Return the version during 'ghcup compile ghc -g <commit>'

During ghcup compile ghc -g <commit> we may auto-discover the GHC version that is being compiled: https://gitlab.haskell.org/haskell/ghcup-hs/-/blob/80e1924e5fc519d1b54471e298efb21bed247929/lib/GHCup.hs#L1564

It would be useful for e.g. stack hooks to know this version, so we can run a subsequent ghcup whereis ghc <ver>.