Use the ghcup nightlies in the GHC head CI

[harendra-kumar]

See https://www.haskell.org/ghcup/guide/#nightlies .

We are currently using this command to install GHC head version: ./ghcup install ghc -u https://gitlab.haskell.org/ghc/ghc/-/jobs/artifacts/master/raw/ghc-x86_64-linux-deb10-int_native-validate.tar.xz?job=x86_64-linux-deb10-int_native-validate head

See https://github.com/composewell/streamly/blob/4e346939a57bc76f3b4a328aecaf3dd17b82035c/.github/workflows/haskell.yml#L211

Is it better to use the ghcup nightlies feature instead?

cc: @hasufell