Announced in [1], versioned tarballs allow to make sure that a
specific version is fetched. This does not guarantee that all previous
versions are retained on the main mirrors.
Logically, we would want to first try to download versioned tarballs
from any mirror and only then try the unversioned ones. But right now
we only have two mirrors and only some of the tarballs are versioned
in texlive-2019, so the order is changed to not hammer the weak
tug.org mirror.
[1] https://tug.org/pipermail/tex-live/2019-September/044086.html
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