bazelisk-plus-incompatible-flags: Gateway Time-out

https://buildkite.com/bazel/bazelisk-plus-incompatible-flags/builds/591#453ea8eb-ade6-4be8-978f-97cdf1217910

The aggregation step fails due to timeouts

```
File "/workdir/bazelci.py", line 599, in _open_url
--
  | urllib.request.urlopen("{}?access_token={}{}".format(url, self._token, params_str))
  | File "/usr/lib/python3.6/urllib/request.py", line 223, in urlopen
  | return opener.open(url, data, timeout)
  | File "/usr/lib/python3.6/urllib/request.py", line 532, in open
  | response = meth(req, response)
  | File "/usr/lib/python3.6/urllib/request.py", line 642, in http_response
  | 'http', request, response, code, msg, hdrs)
  | File "/usr/lib/python3.6/urllib/request.py", line 570, in error
  | return self._call_chain(*args)
  | File "/usr/lib/python3.6/urllib/request.py", line 504, in _call_chain
  | result = func(*args)
  | File "/usr/lib/python3.6/urllib/request.py", line 650, in http_error_default
  | raise HTTPError(req.full_url, code, msg, hdrs, fp)
  | urllib.error.HTTPError: HTTP Error 504: Gateway Time-out
```

The failure is reported in /workdir/bazelci.py at _open_url on line 599; start by reviewing the linked Buildkite aggregation log and the URL-opening call. Investigate the HTTP 504 during aggregation and reproduce the failure; done means the aggregation completes without the gateway timeout.

Written by the indexing model from the issue text.