testing: add an explicit module download step to build_samples.sh (#4291

)

Currently, the work to go mod replace doesn't account for dependency
changes in head.  This adds an explicit step to download modules to
reflect differences in the latest released google-cloud-go vs head.

internal/kokoro/build_samples.sh

@@ -64,6 +64,8 @@ for i in $(find . -name go.mod); do
   go mod edit -replace cloud.google.com/go/pubsub=$gcwd/pubsub
   go mod edit -replace cloud.google.com/go/spanner=$gcwd/spanner
   go mod edit -replace cloud.google.com/go/storage=$gcwd/storage
+  echo "Downloading modules"
+  go mod download
   echo "Building module $i"
   go build ./...
   popd