Switch flag definitions to be on pflag instead of flag in `package go/cmd/zkctl`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/cmd/zkctl/*.go
        zkCfg = flag.String("zk.cfg", "6@<hostname>:3801:3802:3803",
        myID = flag.Uint("zk.myid", 0,
        _flag.SetUsage(flag.CommandLine, _flag.UsageOptions{
                flag.Usage()