Failing on build:sh: artifacts.zip is not a zip file

Hi! @dgoulet asked me to try this out, so here's the first bug report. When I ran build.sh, I got a failure:

$ curl -L -o artifacts.zip https://gitlab.torproject.org/tpo/core/tor-ci-release/-/jobs/artifacts/main/download?job=validation
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 28730    0 28730    0     0  54310      0 --:--:-- --:--:-- --:--:-- 54310
$ unzip -o artifacts.zip
Archive:  artifacts.zip
  End-of-central-directory signature not found.  Either this file is not
  a zipfile, or it constitutes one disk of a multi-part archive.  In the
  latter case the central directory and zipfile comment will be found on
  the last disk(s) of this archive.
unzip:  cannot find zipfile directory in one of artifacts.zip or
        artifacts.zip.zip, and cannot find artifacts.zip.ZIP, period.
FATAL ERROR: command 'unzip -o artifacts.zip' has failed.

When I went to inspect artifacts.zip, I saw that it was a 404 error from gitlab.

I'd suggest that the first fix is to get curl not to store its output on a 404 error; the second fix is to put up the required file, or to make the failure simpler if the file isn't there.