Previously we would use whatever version was in nixpkgs yet this may not
be the version which the patch is supposed to be applied against.

Thanks to vaibhavsagar for the update script [1].

[1] https://github.com/vaibhavsagar/experiments/blob/f3f807212b82f07485c3df041cd8d6236ecb94e2/bump-all-cabal-hashes/updater

- v2-update is preferred (i.e. `v2-` prefix)
- but it's actually broken, due still existing `cabal-install` bug in
  non-append repository updates
- Also add explicit link to haskell.gitlab.com

This fixes the bindist versioning logic.