Fix broken caching of generator data

Stan Hurequested to merge
sh-fix-generators-not-cached into master

This should reduce the slowness of build_branch 1 and the frequency with which we're polling the GitLab API. This merge request:

  1. Breaks out the generation of proxied resources into its own step with its own cache key.
  2. Fixes a bug introduced recently that broke the 24-hour API caching.

This works around an issue where caches on parallel builds don't capture all data across the builds (gitlab-org/gitlab#21409 (closed)).

Closes #5855 (closed)

Edited by Stan Hu

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