flake: add binary cache

Author: asymmetric

README.md

@@ -29,6 +29,9 @@ Install Nix if you haven't already ([instructions](https://nixos.org/download.ht
 nix profile install github:dapphub/dapptools#dapp # or ethsign, hevm, seth
 ```
 
+Nix will offer to use the dapptools binary cache, which will speed up installs,
+but requires you to trust both us and the Cachix infrastructure.
+
 ### Legacy
 
 ```

flake.nix

@@ -7,6 +7,11 @@
     ethereum-hevm.url = "github:ethereum/hevm";
   };
 
+  nixConfig = {
+    extra-substituters = [ "https://dapp.cachix.org" ];
+    extra-substituters-public-keys = [ "dapp.cachix.org-1:9GJt9Ja8IQwR7YW/aF0QvCa6OmjGmsKoZIist0dG+Rs=" ];
+  };
+
   outputs = { self, nixpkgs, ethereum-hevm }:
     let
       supportedSystems = [