udevCheckHook: guard platform

It is technically possible to guard all udevCheckHook usages behind
`lib.optionals (lib.meta.availableOn stdenv.buildPlatform systemdMinimal)`.

However, doing this is hard to read, clunky, and hard to discover.
*Not* doing such a guard would mean cross-compilation darwin -> linux breaks.
The workaround here is to just accept any udev rules if they can't be properly checked.

Author: LordGrimmauld

pkgs/by-name/ud/udevCheckHook/hook.sh

@@ -21,7 +21,7 @@ udevCheckHook() {
     echo Finished udevCheckPhase
 }
 
-if [[ -z "${dontUdevCheck-}" ]]; then
+if [[ -z "${dontUdevCheck-}" && -n "@udevadm@" ]]; then
     echo "Using udevCheckHook"
     preInstallCheckHooks+=(udevCheckHook)
 fi

pkgs/by-name/ud/udevCheckHook/package.nix

@@ -2,12 +2,20 @@
   lib,
   makeSetupHook,
   systemdMinimal,
+  udev,
+  stdenv,
 }:
-
+let
+  # udev rules can only be checked if systemd (specifically, 'udevadm') can be executed on build platform
+  # if udev is not available on hostPlatform, there is no point in checking rules
+  applyHook =
+    lib.meta.availableOn stdenv.hostPlatform udev
+    && lib.meta.availableOn stdenv.buildPlatform systemdMinimal;
+in
 makeSetupHook {
   name = "udev-check-hook";
   substitutions = {
-    udevadm = lib.getExe' systemdMinimal "udevadm";
+    udevadm = if applyHook then lib.getExe' systemdMinimal "udevadm" else "";
   };
   meta = {
     description = "check validity of udev rules in outputs";