verify-vendor: add go mod tidy

A recent commit passed the CI test for go mod tidy but introduced a bug
in master when we run go mod tidy. By including go mod tidy, we can
catch these earlier.

hack/verify-vendor.sh

@@ -2,6 +2,7 @@
 
 if [ "$IS_CONTAINER" != "" ]; then
   set -euxo pipefail
+  go mod tidy
   go mod vendor
   go mod verify
   git diff --exit-code