Nightly: fetch the release over 16 connections

The first publish spent 9 min 42 s in the download action for the 4.6 GB
stable release; the 11.5 GB edge release would not have fitted in the
job's 45 minutes. GitHub's artifact storage limits each connection to a
few megabytes a second, and the action uses few.

An artifact is one zip at a signed address. test/vm-image/nightly-fetch
asks the API for that address and takes it with aria2c over 16
connections, checks the size and unpacks it. The same 4.6 GB artifact
came down in 123 s that way. The publish job gets actions: read for it,
and checks out test/vm-image for the script.

test/unit/nightly-test.sh plays the network with stand-ins: the newest
artifact of a name is the one fetched, a failed download is tried again
with a new address, a wrong size is refused and not unpacked.

.github/workflows/unit-test.yml

@@ -50,7 +50,7 @@ jobs:
             # mirror that drops one file must not fail the tests.
             sed -i "1i Server = https://mirror.omarchy.org/\$repo/os/\$arch" /etc/pacman.d/mirrorlist
             for attempt in 1 2 3; do
-              pacman -Syu --noconfirm --needed python parted diffutils jq >/dev/null && break
+              pacman -Syu --noconfirm --needed python parted diffutils jq zip unzip >/dev/null && break
               [ "$attempt" -lt 3 ] || { echo "::error::the test container could not install its tools"; exit 1; }
               sleep 15
             done