Resolve "Test/Production environment imparity in nokogiri"

What does this MR do?

This avoids the manual install of the fog-aws and mime-types gems for other jobs than the one that needs it (update-knapsack).

Are there points in the code the reviewer needs to double check?

I've added a step to the knapsack job to retrieve the report file, to ensure this change doesn't break the update-knapsack job / scripts/sync-reports script (https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/22358148). This change has been removed since then.

Why was this MR needed?

Because manually installing fog-aws leads to overwriting the Nokogiri (and probably other gems) version installed with Bundler.

Also, by not installing these gems in the setup-test-env job should save a few seconds.

Closes #34915 (closed)