Fix artifact-prefixed uploads with empty media root - #8286

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Failed to save: Unable to reserve cache with key setup-uv-2-x86_64-unknown-linux-gnu-ubuntu-24.04-3.14.9-pruned-e1cbf60e380c571e9abfb22fa93575101240b3fd24ca30edcd90311219d1234e, another job may be creating this cache.

Cache reservation failed: cache write denied: token has no writable scopes