Switch flag definitions to be on pflag instead of flag in `package go/vt/vttablet/customrule/filecustomrule`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/vttablet/customrule/filecustomrule/*.go
go/vt/vttablet/customrule/filecustomrule/filecustomrule.go:     fileRulePath = flag.String("filecustomrules", "", "file based custom rule path")
go/vt/vttablet/customrule/filecustomrule/filecustomrule.go:     fileRuleShouldWatch = flag.Bool("filecustomrules_watch", false, "set up a watch on the target file and reload query rules when it changes")