Update ghc-nix cachix public key

Locally, I got

warning: ignoring substitute for '/nix/store/rmkg4dvdsbwgzydlfn06lvdq8m3sisqf-source' from 'https://ghc-nix.cachix.org', as it's not signed by any of the keys in 'trusted-public-keys'

I got the new key from https://app.cachix.org/cache/ghc-nix; it should probably be confirmed that this change is expected.