CI failure on ubuntu-latest

[Gau-thier]

Summary

CI on ubuntu-latest is failing for the following reason:

Fatal error: exception Unix.Unix_error(Unix.EADDRINUSE, "bind", "")
Raised by primitive operation at Lwt_unix.bind in file "src/unix/lwt_unix.cppo.ml", line [18](https://github.com/marigold-dev/pollinate/runs/5642877027?check_suite_focus=true#step:6:18)50, characters 52-74
Called from Pollinate__Util.Net.create_socket in file "lib/util.ml", line 40, characters 17-32
Called from Pollinate__Client.init in file "lib/client.ml", line 67, characters [19](https://github.com/marigold-dev/pollinate/runs/5642877027?check_suite_focus=true#step:6:19)-41
Called from Dune__exe__Commons.Commons.client_a in file "test/commons.ml", line 41, characters 17-80
(cd _build/default/test && ./client_tests.exe)

While the same Github Action is ✅ on macos-latest.

Already done

• Force actions to don't be parallel => same result 🚫
• Change client port numbers from 300X to 500X => same result 🚫