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: 93f780c
GitHub-Pull-Request: #23
Reviewed-on: https://go-review.googlesource.com/c/build/+/193597
Reviewed-by: Andrew Bonventre <[email protected]>
Reviewed-by: Alexander Rakoczy <[email protected]>

Author: dmitshur

app/appengine/README.md

@@ -20,11 +20,11 @@ dev_appserver.py --enable_host_checking=false --host=0.0.0.0 --port=8080 .
 
 ```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 [email protected] --project=golang-org deploy app.yaml
+GO111MODULE=on gcloud app [email protected] --project=golang-org deploy app.yaml
 ```