Refactory flag.commandline to use our own flag.FlagSet instead

[ying-jeanne]

when update to cuelang.org/go v0.4.0 we got

go build -ldflags -w -X main.version=8.3.0-pre -X main.commit=1d56c32ab0 -X main.buildstamp=1632311421 -X main.buildBranch=cueupdate -o ./bin/grafana-server ./pkg/cmd/grafana-server
[Bra] 09-22 14:20:47 [DEBUG] Running: ./bin/grafana-server [-packaging=dev cfg:app_mode=development]
./bin/grafana-server flag redefined: v
panic: ./bin/grafana-server flag redefined: v

there is a ticket opened at cuelang suggest the work around and best practice.
cue-lang/cue#1199

we need to fix this in order to make cuetsy integrated into Grafana later on.