ci: Remove assets from native cache when assets package exists

What does this MR do and why?

This should fix gitlab-org/quality/engineering-productivity/team#101 (closed), where outdated assets from native cache would be used.

We can see that the number of files under public/assets that are saved as artifacts is exactly the same as the number of files from the compiled assets package:

https://gitlab.com/gitlab-org/gitlab/-/jobs/3318908536#L721:

public/assets/: found 4960 matching files and directories 

Number of files in the archive:

$ find ~/Downloads/assets-test-ee-e8216b6e493faeb7359ada661cebcdbdc6a9bf91566187d3d667e382e7bf42be-v2/public/assets | wc -l 
    4960

MR acceptance checklist

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