nixos/tests/grafana-agent: update port

We now don't explicitly configure a self-chosen port, but use the
default port choosen by grafana, 12345.

nixos/tests/grafana-agent.nix

@@ -23,9 +23,9 @@ import ./make-test-python.nix ({ lib, pkgs, ... }:
 
       with subtest("Grafana-agent is running"):
           machine.wait_for_unit("grafana-agent.service")
-          machine.wait_for_open_port(9090)
+          machine.wait_for_open_port(12345)
           machine.succeed(
-              "curl -sSfN http://127.0.0.1:9090/-/healthy"
+              "curl -sSfN http://127.0.0.1:12345/-/healthy"
           )
           machine.shutdown()
     '';