sysutils/kubectl: Switch to USES=go:modules

Fix build with go1.13beta1

> Building for kubectl-1.15.0

go: bitbucket.org/bertimus9/systemstat@v0.0.0-20180207000608-0eeff89b0690: Get https://proxy.golang.org/bitbucket.org/bertimus9/systemstat/@v/v0.0.0-20180207000608-0eeff89b0690.mod: dial tcp: lookup proxy.golang.org on 1.0.0.1:5
3: write udp 127.0.0.1:27372->1.0.0.1:53: write: permission denied

• Error code 1

PR: 238946
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: danilo (maintainer timeout, 23 days)