publish master CI job does not support Go 1.13

[bwplotka]

https://circleci.com/gh/thanos-io/thanos/6396?utm_campaign=workflow-failed&utm_medium=email&utm_source=notification

# github.com/thanos-io/thanos/pkg/block
pkg/block/fetcher.go:181:7: undefined: "errors".Is
note: module requires Go 1.13
!! command failed: build -o /go/src/github.com/thanos-io/thanos/thanos -ldflags -X github.com/prometheus/common/version.Version=0.9.0-dev -X github.com/prometheus/common/version.Revision=f024461d43813ac3eba24e3d69d391792b98188a -X github.com/prometheus/common/version.Branch=master -X github.com/prometheus/common/version.BuildUser=circleci@aced7b3b3ad5 -X github.com/prometheus/common/version.BuildDate=20200106-12:40:16  -extldflags '-static' -a -tags netgo github.com/thanos-io/thanos/cmd/thanos: exit status 2
Makefile:138: recipe for target 'build' failed

Investigating.