Cache go mod downloads for setup-test-env

What does this MR do?

The setup-test-env job has to build a number of Golang dependencies, including gitlab-elasticsearch-indexer. We should cache the $GOPATH/pkg/mod file across builds to avoid downloading the same files over and over and over again.

In particular, we're seeing elevated error rates between runners and github.com right now, leading to many failed builds.

Closes #32433 (closed)