talks: set CHECK_COUNTRY env var in app.yaml

This uses the new golangorgenv package from CL 165459 to enable
country check in the /share endpoint registered by x/tools/playground
package. This improves consistency with other similar services that
also use the x/tools/playground package.

Updates golang/go#30486

Change-Id: Idc6db4e384a7392aa39967f3320c2158a9c7e99a
Reviewed-on: https://go-review.googlesource.com/c/talks/+/165742
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

app.yaml

diff --git a/app.yaml b/app.yaml
index c835cbf..464e028 100644
--- a/app.yaml
+++ b/app.yaml
@@ -1,6 +1,9 @@
 service: talks
 runtime: go111
 
+env_variables:
+  GOLANGORG_CHECK_COUNTRY: true
+
 handlers:
 - url: /favicon.ico
   static_files: static/favicon.ico