nixos/udev: silence harmless warnings

`udevRulesFor` generates a lot of warnings like:

  substituteStream(): WARNING: pattern '"/sbin/modprobe' doesn't match anything in file '/nix/store/.../95-dm-notify.rules'

due to the (preemptive) substitution of common paths in the default
udev rules. In this case a file having no matches is not unepected
and poses no issue at all.

nixos/modules/services/hardware/udev.nix

@@ -72,7 +72,7 @@ let
           --replace \"/sbin/blkid \"${pkgs.util-linux}/sbin/blkid \
           --replace \"/bin/mount \"${pkgs.util-linux}/bin/mount \
           --replace /usr/bin/readlink ${pkgs.coreutils}/bin/readlink \
-          --replace /usr/bin/basename ${pkgs.coreutils}/bin/basename
+          --replace /usr/bin/basename ${pkgs.coreutils}/bin/basename 2>/dev/null
       ${optionalString (initrdBin != null) ''
         substituteInPlace $i --replace '/run/current-system/systemd' "${removeSuffix "/bin" initrdBin}"
       ''}