ghc-cabal: force use of UTF8 when writing out `haddock-prologue.txt`

This unbreaks the GHC build if a non-UTF8 locale such as LANG=C is active

See also haskell/cabal#1721 and haskell/haddock#286

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>