app/appengine: set GO111MODULE=on for deploy

Deploying in GOPATH mode is not supported, so explicitly specify
GO111MODULE=on to use module mode regardless of local setup.

Fixes golang/go#34107

Change-Id: I5bc2518c4cd4c329eea65684fec96aae5b0bf808
GitHub-Last-Rev: 93f780cd584e74e5c63cb9b49260b33722b5c9a3
GitHub-Pull-Request: golang/build#23
Reviewed-on: https://go-review.googlesource.com/c/build/+/193597
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
diff --git a/app/appengine/README.md b/app/appengine/README.md
index 47dd42e..d4330d9 100644
--- a/app/appengine/README.md
+++ b/app/appengine/README.md
@@ -20,11 +20,11 @@
 
 ```sh
 gcloud config set project golang-org
-gcloud app deploy app.yaml
+GO111MODULE=on gcloud app deploy app.yaml
 ```
 
 or, to not affect your gcloud state, use:
 
 ```
-gcloud app --account=username@google.com --project=golang-org deploy app.yaml
+GO111MODULE=on gcloud app --account=username@google.com --project=golang-org deploy app.yaml
 ```