mpd: set network settings as environment variables

https://mpd.readthedocs.io/en/stable/client.html#connecting-to-mpd

These environment variables are picked up by, amongst others, clients
based on libmpdclient:

- 2a42a10067/src/settings.c (L108)
- 2a42a10067/src/settings.c (L135)

modules/misc/news/2026/01/2026-01-02_00-03-48.nix

@@ -0,0 +1,10 @@
+{ config, ... }:
+{
+  time = "2026-01-02T00:03:48+00:00";
+  condition = config.services.mpd.enable;
+  message = ''
+    `MPD_HOST` and `MPD_PORT` environment variables are now set automatically.
+
+    This can be disabled with `services.mpd.enableSessionVariables = false`.
+  '';
+}