Run update-cache jobs after prepare stage

What does this MR do?

Fix the error seen in https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/pipelines/463194744, which was caused by !5877 (merged)

1. Ensure cache update jobs are run after prepare stage (fetch-assets)
2. Mark explicit needs on fetch-assets for update-trigger-package-cache

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened