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

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/wrangler/*.go
go/vt/wrangler/vdiff_env_test.go:       flag.Set("tablet_protocol", "VDiffTest")
go/vt/wrangler/wrangler_env_test.go:    flag.Set("tablet_protocol", "WranglerTest")

[ajm188]

How we tackle this one is going to depend on how exactly we implement #10978