ci(unit-tests): the tshark apt fetch of libwireshark17t64 still stalls past the 180s bound on both attempts #1432

Describe the bug
The tshark apt install still stalls past its per-attempt bound. #1306 raised that bound to 180s. On 2026-10-08 the 23.7 MB libwireshark17t64 fetch from azure.archive.ubuntu.com was killed by timeout 180 on both attempts, three times:

Each run's earlier packages also crawled (0.5–4s for 35–450 kB), while the index fetch was fast. Reruns on other heads took 12–17s.

Expected behavior
A slow mirror should not turn main or a PR red. The comment at unit-tests.yml:515-527 (from #974) already rules out Acquire::Retries and http::Timeout, which are noble's defaults.

Options that change the outcome:

• cache the downloaded .deb files with actions/cache, keyed on the package versions
• a fallback mirror on the retry
• a larger bound, within the 10-minute step cap