Build instructions in README don’t work

[int-index]

Instructions in the ghc-9.0 branch are out of date. They say:

cabal v2-build -w ghc-8.10.1
cabal v2-test -w ghc-8.10.1 all

But I had to do:

ghcup set ghc 9.0.0.20201227
ghcup set cabal 3.4.0.0-rc4
cabal build --allow-newer="Cabal"

The --allow-newer is needed because of ghc-paths, see simonmar/ghc-paths#24

[int-index]

Fixed by fcf9a04