Fix obselate nix/README.md section

[glyh]

https://github.com/MinaProtocol/mina/tree/compatible/nix#warning-ignoring-untrusted-substituter

This public key is broken and it seems the fix is to enable

accept-flake-config = true

in nix.conf, but isn't this enforcing use of flake, then? I've no clue as I'm not a Nix hacker so I'd ping Nix users here.

@georgeee @cjjdespres

[aleskrin]

Hi team @glyh @georgeee @cjjdespres I’d love to work on this Nix-related fix. Could you please assign it to me?

[glyh]

There you go :) Thanks for the initiative