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.