bitcoind: use type str for rpcbind

Extra RPC bind addresses can still be added via extraConfig.

modules/electrs.nix

@@ -97,7 +97,7 @@ in {
           --daemon-dir='${bitcoind.dataDir}' \
           --electrum-rpc-addr=${cfg.address}:${toString cfg.port} \
           --monitoring-addr=${cfg.address}:${toString cfg.monitoringPort} \
-          --daemon-rpc-addr=${builtins.elemAt bitcoind.rpcbind 0}:${toString bitcoind.rpc.port} \
+          --daemon-rpc-addr=${bitcoind.rpcbind}:${toString bitcoind.rpc.port} \
           ${cfg.extraArgs}
         '';
         User = cfg.user;