busybox: lower priority to 15, below systemd and coreutils

Busybox, systemd and coreutils all have priority 10. Busybox binaries
have been observed to shadow systemd and coreutils binaries.

If systemd is used, its binaries should be preferred, as they are aware
of systemd. For instance, the busybox provided `reboot` cannot reboot
NVidia Jetson AGX Xavier, whereas the systemd `reboot` can.

This also gives busybox lower priority than coreutils. Busybox is meant
for embedded systems with limited resources. If busybox and coreutils
coexist, the resources are normally not an issue. Busybox should only
shadow binaries that are known to be deficient.

If anyone wants to prefer busybox (e.g. memory constrained system with
plenty of non-volatile storage), it's up to them to prioritize busybox
manually above coreutils, util-linux and other packages. It's not a
common case.

pkgs/os-specific/linux/busybox/default.nix

@@ -167,6 +167,6 @@ stdenv.mkDerivation rec {
     license = licenses.gpl2Only;
     maintainers = with maintainers; [ TethysSvensson qyliss ];
     platforms = platforms.linux;
-    priority = 10;
+    priority = 15; # below systemd (halt, init, poweroff, reboot) and coreutils
   };
 }