module: override metrics & http address for worker

Closes #72

So, #72 is about a segfault in the LDAP outpost, but this is the actual
culprit[0]:

* Both server & worker share the same configuration in this setup.

* Since 2025.8 this means that both try to start a server for metrics at
  port 9300 and an HTTP server (in the worker case for healthchecks) at
  port 9000.

* On upgrades, migrations are performed. Only the server waited for the
  migrations to finish, hence the worker started up earlier. As a
  result, it was quicker in binding port 9000 in ONLY this case (and
  thus, this was never reproducible on a second attempt!). Now, on port
  9000 was NOT the authentik server, but something that returned an
  empty response for everything that's not the healthcheck.

* As a result, the LDAP outpost got a response from what it believed was
  authentik, but actually `nil, nil` because of the empty response.
  Trying to dereference values from that response[1] caused the
  segfault.

The fix is pretty easy, just override the listen ports via the
environment. Unfortunately, the docs[2] are apparently not entirely correct[3],
given the Python code it must be LISTEN__LISTEN_HTTP[4]. I added a
test-case to ensure that the config is properly applied.

[0] Reported as https://github.com/goauthentik/authentik/issues/16850
[1] 57e12cef06/internal/outpost/ak/api.go (L95)
[2] https://docs.goauthentik.io/install-config/configuration/#listen-settings
[3] Reported as https://github.com/goauthentik/authentik/issues/16851
[4] 57e12cef06/authentik/lib/config.py (L238)

tests/minimal-vmtest.nix

@@ -91,5 +91,12 @@ pkgs.nixosTest {
         machine.succeed("su - alice -c 'firefox http://localhost/' >&2 &")
         machine.wait_for_text("authentik")
         machine.screenshot("5_nginx_proxies_requests")
+
+    with subtest("metrics & worker"):
+        machine.wait_for_open_port(9300)
+        machine.wait_for_open_port(9301)
+
+        print(machine.succeed("curl -L localhost:9300/metrics | grep authentik_outpost_connection | grep 'Embedded'"))
+        print(machine.succeed("curl -L localhost:9301/metrics | grep authentik_tasks_total"))
   '';
 }