erigon: add package option to override default version

Signed-off-by: Jakub Sokołowski <[e-mail]>

nixos/modules/services/blockchain/ethereum/erigon.nix

···
     services.erigon = {
       enable = mkEnableOption (lib.mdDoc "Ethereum implementation on the efficiency frontier");
 
+      package = mkPackageOptionMD pkgs "erigon" { };
+
       extraArgs = mkOption {
         type = types.listOf types.str;
         description = lib.mdDoc "Additional arguments passed to Erigon";
···
 
       serviceConfig = {
         LoadCredential = "ERIGON_JWT:${cfg.secretJwtPath}";
-        ExecStart = "${pkgs.erigon}/bin/erigon --config ${configFile} --authrpc.jwtsecret=%d/ERIGON_JWT ${lib.escapeShellArgs cfg.extraArgs}";
+        ExecStart = "${cfg.package}/bin/erigon --config ${configFile} --authrpc.jwtsecret=%d/ERIGON_JWT ${lib.escapeShellArgs cfg.extraArgs}";
         DynamicUser = true;
         Restart = "on-failure";
         StateDirectory = "erigon";