[cli] Use pflag/flag interop function in vtctldclient legacy shim (#11399)

Closes #11284.

Signed-off-by: Andrew Mason <andrew@planetscale.com>

go/cmd/vtctldclient/command/legacy_shim.go

@@ -18,13 +18,13 @@ package command
 
 import (
 	"context"
-	"flag"
 	"fmt"
 	"strings"
 
 	"github.com/spf13/cobra"
 
 	"vitess.io/vitess/go/cmd/vtctldclient/cli"
+	"vitess.io/vitess/go/internal/flag"
 	"vitess.io/vitess/go/vt/log"
 	"vitess.io/vitess/go/vt/logutil"
 	"vitess.io/vitess/go/vt/vtctl/vtctlclient"