deploy: add explicit GOPROXY

The PostValidate step is failing with an error similar to that
seen in https://github.com/golang/go/issues/48713.

Try adding an explicit GOPROXY env variable to fix (this was the
workaround suggested in that bug).

Change-Id: Ief36255e1668f89400593b63d3063f96a9cf9057
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/551835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/deploy/build.yaml b/deploy/build.yaml
index ace5047..5531c65 100644
--- a/deploy/build.yaml
+++ b/deploy/build.yaml
@@ -69,3 +69,5 @@
     name: golang:1.12.5
     entrypoint: bash
     args: ["-ec", "go run ./cmd/checkdb -v1 -legacy /workspace/deployed/go-vulndb"]
+    env:
+      - 'GOPROXY=https://proxy.golang.org'