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

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/vttablet/customrule/topocustomrule/*.go
go/vt/vttablet/customrule/topocustomrule/topocustomrule.go:     ruleCell = flag.String("topocustomrule_cell", "global", "topo cell for customrules file.")
go/vt/vttablet/customrule/topocustomrule/topocustomrule.go:     rulePath = flag.String("topocustomrule_path", "", "path for customrules file. Disabled if empty.")

[rsajwani]

go list -f '{{ .ImportPath }}| {{ join .Deps " " }} ' ./go/cmd/... | awk -F"|" '$2 ~ "vitess.io/vitess/go/vt/vttablet/customrule/topocustomrule" {print $1}'
vitess.io/vitess/go/cmd/vttablet