Switch flag definitions to be on pflag instead of flag in `package go/vt/workflow`

[ajm188]

Part of https://github.com/vitessio/vitess/issues/10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/workflow/*.go
go/vt/workflow/sleep_workflow.go:       subFlags := flag.NewFlagSet(sleepFactoryName, flag.ContinueOnError)
go/vt/workflow/workflow_test.go:        subFlags := flag.NewFlagSet(testWorkflowFactoryName, flag.ContinueOnError)

[ajm188]

I think we can just ignore this one, provided this code is definitely going to be deleted before v15, is that correct @deepthi ?