Fix GOOS/GOARCH variables not being present in cross.
Fix GOOS/GOARCH variables not being present in cross.

See comment. So annoying.

matt-deboer/golang-builder,argvader/golang-builder,rupakg/golang-builder,CenturyLinkLabs/golang-builder,alpe/golang-builder,tryggth/golang-builder,patocox/golang-builder,GoogleCloudPlatform/golang-builder,ethernetdan/golang-builder