Backport 'Enable assets caching on stable branches'

What does this MR do and why?

Backporting !196480 (merged)

It's to enable production assets caching jobs (cache-assets) in canonical stable branch commit pipelines, so that compile-production-assets jobs in security doesn't need to recompile with every commit, when the canonical stable branch commits contain the same assets.

We backported to 18-1-stable-ee, but forgot to backport it to 18-0-stable-ee.

Related: gitlab-com/gl-infra/delivery#20964 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• [-] This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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