cmd/releasebot: set GO111MODULE=off for go get .../cmd/release

We specifically create a temporary GOPATH workspace in order to run
'go get golang.org/x/build/cmd/release' to install the release binary.
This was done with the intention to use GOPATH mode for the build.

Specify GO111MODULE=off explicitly so that GOPATH mode is always
used, regardless of local user configuration. This will produce
more consistent and reproducible results.

Installing the release binary in module mode is future work.

Fixes golang/go#33031

Change-Id: I1df009432be3d0af4e5e7679f38d147901d950e7
Reviewed-on: https://go-review.googlesource.com/c/build/+/193720
Reviewed-by: Alexander Rakoczy <alex@golang.org>

cmd/releasebot/main.go

@@ -564,7 +564,7 @@ func (w *Work) buildReleaseBinary() {
 	if err := os.MkdirAll(gopath, 0777); err != nil {
 		w.log.Panic(err)
 	}
-	r := w.runner(w.Dir, "GOPATH="+gopath, "GOBIN="+filepath.Join(gopath, "bin"))
+	r := w.runner(w.Dir, "GO111MODULE=off", "GOPATH="+gopath, "GOBIN="+filepath.Join(gopath, "bin"))
 	r.run("go", "get", "golang.org/x/build/cmd/release")
 	w.ReleaseBinary = filepath.Join(gopath, "bin/release")
 }