Tezt/Cloud: Fix an issue for register_binary with proxy mode

The agents from the agents function does not take into the proxy agent. But this is necessary for introspection. We fix that by looking at all the agents taking directly the agent field from the cloud datatype.

I was able to check it fixed the issue by running:

dune exec tezt/tests/cloud/main.exe -- cloud dal -v --log-file /tmp/log --network ghostnet --vms-limit 0 --bootstrap --monitoring --proxy --proxy-localhost --tezt-cloud baker-ghostnet --dockerfile-alias saroupille

If you want to try, you can probably check it works as expected using:

dune exec tezt/tests/cloud/main.exe -- cloud dal -v --log-file /tmp/log --vms-limit 0 --bootstrap --monitoring --proxy --proxy-localhost --producers 1