dashboard/env: add a cgo-disabled Linux builder
dashboard/env: add a cgo-disabled Linux builder

I think this is all that's needed?

Fixes golang/go#4795

LGTM=adg, dave
R=adg, dave
CC=golang-codereviews
https://golang.org/cl/138090043

golang/build,golang/build,evandbrown/build,golang/build,evandbrown/build,proppy/build,evandbrown/build,proppy/build,proppy/build,golang/build,proppy/build