ci: add cache key based on go.mod file

What does this MR do?

See https://docs.gitlab.com/ee/ci/yaml/index.html#cachekeyfiles

Add a cache key based on the go.mod file to the download deps job to reuse some caches and rebuild it less often.

This is part of an effort to reduce CI minutes consumption in GitLab products.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines