etcdctl: unset ETCDCTL_ARGS on cov builds
etcdctl: unset ETCDCTL_ARGS on cov builds

The stricter warnings on pkg/flags generates extra output that
break coverage tests. Unset the ETCDCTL_ARGS environment variable
so the warnings aren't printed.

visheshnp/etcd,lorneli/etcd,xiang90/etcd,xiang90/etcd,mqliang/etcd,etcd-io/etcd,sinsharat/etcd,rnaveiras/etcd,heyitsanthony/etcd,rnaveiras/etcd,gyuho/etcd,akauppi/etcd,whuwxl/etcd,li-ang/etcd,dannysauer/etcd,bdarnell/etcd,whuwxl/etcd,ajityagaty/etcd,akauppi/etcd,sebrandon1/etcd,bdarnell/etcd,visheshnp/etcd,mickep76/etcd,sinsharat/etcd,mkumatag/etcd,bluebreezecf/etcd,coreos/etcd,bluebreezecf/etcd,li-ang/etcd,mkumatag/etcd,dannysauer/etcd,sebrandon1/etcd,fanminshi/etcd,a-robinson/etcd,joshix/etcd,ajityagaty/etcd,joshix/etcd,a-robinson/etcd,mqliang/etcd,gyuho/etcd,heyitsanthony/etcd,vimalk78/etcd,glevand/coreos--etcd,glevand/coreos--etcd,rnaveiras/etcd,lorneli/etcd,YuleiXiao/etcd,mqliang/etcd,etcd-io/etcd,YuleiXiao/etcd,mickep76/etcd,mkumatag/etcd,cockroachdb/etcd,etcd-io/etcd,vimalk78/etcd,coreos/etcd,fanminshi/etcd,ajityagaty/etcd,cockroachdb/etcd