cmd/utils: fix NetworkId default when -dev is set (ethereum#16833)

Prior to this change, when geth was started with `geth -dev -rpc`,
it would report a network id of `1` in response to the `net_version` RPC
request. But the actual network id it used to verify transactions
was `1337`.

This change causes geth instead respond with `1337` to the `net_version`
RPC when geth is started with `geth -dev -rpc`.

cmd/utils/flags.go

@@ -1084,6 +1084,9 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *eth.Config) {
 		}
 		cfg.Genesis = core.DefaultRinkebyGenesisBlock()
 	case ctx.GlobalBool(DeveloperFlag.Name):
+		if !ctx.GlobalIsSet(NetworkIdFlag.Name) {
+			cfg.NetworkId = 1337
+		}
 		// Create new developer account or reuse existing one
 		var (
 			developer accounts.Account