GO111MODULE needs to be set when building pillar

[deitch]

Description

When run in a docker container, we explicitly move out of the GOPATH to build. But when run locally, we run into build issues.

Steps to reproduce the issue:

go get github.com/lf-edge/eve
cd $GOPATH/src/github.com/lf-edge/eve
cd pkg/pillar
make zedbox

Describe the results you received:

Building dist/amd64/zedbox
go build -mod=vendor -ldflags -X=main.Version=-70860ff8-2019-05-22.09.52 -o dist/amd64/zedbox ./zedbox
build flag -mod=vendor only valid when using modules
make: *** [dist/amd64/zedbox] Error 1

Describe the results you expected:

It should build

Additional information you deem important (e.g. issue happens only occasionally):

Will open a PR to fix it.