nixos/rngd: fix clean shutdown

It seems disabling DefaultDependencies
removes these implicit dependencies [0] that
we needed for shutdown to happen cleanly.

Fixes #80871

[0]: https://www.freedesktop.org/software/systemd/man/systemd.service.html#Default%20Dependencies

nixos/modules/security/rngd.nix

@@ -37,6 +37,13 @@ in
 
       after = [ "dev-random.device" ];
 
+      # Clean shutdown without DefaultDependencies
+      conflicts = [ "shutdown.target" ];
+      before = [
+        "sysinit.target"
+        "shutdown.target"
+      ];
+
       description = "Hardware RNG Entropy Gatherer Daemon";
 
       # rngd may have to start early to avoid entropy starvation during boot with encrypted swap