nixos/tests: fix guix under network-online dep fix

Changed files
+1
nixos
tests
+1
nixos/tests/guix/publish.nix
···
# Now it's the client turn to make use of it.
substitute_server = "http://server.local:${toString publishPort}"
client.wait_for_unit("network-online.target")
response = client.succeed(f"curl {substitute_server}")
assert "Guix Substitute Server" in response
···
# Now it's the client turn to make use of it.
substitute_server = "http://server.local:${toString publishPort}"
+
client.systemctl("start network-online.target")
client.wait_for_unit("network-online.target")
response = client.succeed(f"curl {substitute_server}")
assert "Guix Substitute Server" in response