#3946 ci(linux): reword the deb-copy guard comment

Applies the non-blocking wording note from the #3939 review: the guard comment now reads "An unmatched glob stays literal and fails cp under set -e when the runner image already has every package." Comment-only change in .github/workflows/linux.yml.

The reviewer's other note (the save step may store an empty cache entry when the guard skips) is harmless, since apt downloads the packages again, so it stays as is.

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