source-tarball nightly job is failing

See: https://gitlab.haskell.org/ghc/ghc/-/jobs/1007123

| Run Win32Tarballs ListTarballs: none
Traceback (most recent call last):
  File "/builds/ghc/ghc/mk/get-win32-tarballs.py", line 75, in <module>
    main()
  File "/builds/ghc/ghc/mk/get-win32-tarballs.py", line 70, in main
    action(arch)
  File "/builds/ghc/ghc/mk/get-win32-tarballs.py", line 44, in list_arch
    req = urllib.request.urlopen(manifest_url)
  File "/usr/lib/python3.7/urllib/request.py", line 222, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.7/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/usr/lib/python3.7/urllib/request.py", line 641, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.7/urllib/request.py", line 569, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.7/urllib/request.py", line 503, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.7/urllib/request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 404: Not Found
Command line: /usr/bin/python3 /builds/ghc/ghc/mk/get-win32-tarballs.py list all
===> Command failed with error code: 1
Command failed
Build failed.