go.tools/dashboard/builder: ignore GOPATH setting during build (on windows)
go.tools/dashboard/builder: ignore GOPATH setting during build (on windows)

R=golang-dev, minux.ma, adg
CC=golang-dev
https://golang.org/cl/18920046

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