nixosTests.airsonic: fix failure (type error)

airsonic_is_up should return a bool, but machine.succeed returns a
string causing testScriptWithTypes to fail. This is fixed by executing
the cmd with machine.execute and checking the status code.

nixos/tests/airsonic.nix

···
 
   testScript = ''
     def airsonic_is_up(_) -> bool:
-        return machine.succeed("curl --fail http://localhost:4040/login")
+        status, _ = machine.execute("curl --fail http://localhost:4040/login")
+        return status == 0
 
 
     machine.start()