Clean up `flake.nix`

[bfeitknecht]

The deploy workflow relies on flake.nix's provided build attribute which is currently kinda hacky and probably not optimal.

[bfeitknecht]

evaluation warning: nixfmt-rfc-style is now the same as pkgs.nixfmt which should be used instead.

[bfeitknecht]

do you want to allow configuration setting 'extra-substituters' to be set to 'https://ghc-wasm-meta.cachix.org' (y/N)? y
do you want to permanently mark this value as trusted (y/N)? y
do you want to allow configuration setting 'extra-trusted-public-keys' to be set to 'ghc-wasm-meta.cachix.org-1:6C9S967v6XvAizKovS/I844z6o0qR2r3/2DkY7vXGgY=' (y/N)? y
do you want to permanently mark this value as trusted (y/N)? y
warning:
         … during download of 'https://ghc-wasm-meta.cachix.org/nix-cache-info'

         warning: unable to download 'https://ghc-wasm-meta.cachix.org/nix-cache-info': HTTP error 401

         response body:

         Binary cache ghc-wasm-meta doesn't exist or you're not authorized to access. Please check the name for typos and/or provide an auth token. Contact support on https://cachix.org if you're stuck!

[bfeitknecht]

Done in d3022b8.