ci: Extract assets 'tmp/cache' to a separate native cache
What does this MR do and why?
This is to ensure that the component caching mechanism doesn't store these files. These files aren't compiled assets, and can be reused partially if present so, it's ok to use native cache for those.
This should reduce the size of the assets packages, from 218MB to 171MB, see the following screenshot of the same package with or without tmp/cache: 
This extraction from the main native assets cache is needed in the case we want to stop using this main native assets cache in the future.
This will also save approximately 1 minute and 28 seconds, even for MRs that touches frontend files.
Testing matrix
| Assets cache (via package) | Assets tmp cache | Job | Timing | Notes |
|---|---|---|---|---|
| Absent | Absent | https://gitlab.com/gitlab-org/gitlab/-/jobs/3126952898 | 7m 34s | |
| Absent | Present | https://gitlab.com/gitlab-org/gitlab/-/jobs/3127113730 | 6m 6s | We save 1 minute and 28 seconds by having a fresh tmp/cache! |
| Fresh (cache job: https://gitlab.com/gitlab-org/gitlab/-/jobs/3127195842) | Absent | https://gitlab.com/gitlab-org/gitlab/-/jobs/3127287482 | 2m 36s | This doesn't change much since assets are already compiled, we skip compilation anyway. |
| Fresh (cache job: https://gitlab.com/gitlab-org/gitlab/-/jobs/3127195842) | Present (cache job: https://gitlab.com/gitlab-org/gitlab/-/jobs/3127014412) | https://gitlab.com/gitlab-org/gitlab/-/jobs/3127235289 | 2m 20s | This doesn't change much since assets are already compiled, we skip compilation anyway. |
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #375485 (closed).