deploy: upload screentest artifacts

Change-Id: Id0eeb2041bac6a9de7ec32ffeeefaf681ed6be16
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/400997
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Auto-Submit: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/deploy/deploy.yaml b/deploy/deploy.yaml
index 28d4755..312850b 100644
--- a/deploy/deploy.yaml
+++ b/deploy/deploy.yaml
@@ -92,3 +92,7 @@
     entrypoint:  deploy/pagecheck.sh
     args:
       - beta
+artifacts:
+  objects:
+    location: 'gs://$PROJECT_ID/screentest-artifacts'
+    paths: ['tests/screentest/testdata/*.png']
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