Triggered build should use caching

Related to https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2234

When build is triggered from CE https://gitlab.com/gitlab-org/gitlab-ce/builds/15661103, the build machines does not have caching. This means we will build the package from scratch every time. We should look into creating cache for these builds.

/cc @gitlab-build-team

mentioned in issue #2322

This requires an amazon bucket to store the cache and a dedicated runner for triggered builds that can be linked to this bucket.

Not necessarily. You could simply provide two sets of credentials, AFAIK, and set the right values for use with triggered builds.

mentioned in issue #2461

changed milestone to %10.0

added Scheduled label