Go 1.5 doesn't permit our use of GOBIN anymore, breaking cross-compilation (for details, see https://github.com/golang/go/issues/9769) In retrospect, I'm not sure why we set GOBIN in the first place; GOPATH/bin should suffice. Closes #2337
This way, a template can overwrite do_build but still have good defaults for GOPATH, GOARCH and GOARM.