nixos/sd-image: explicit reference to the gawk package

The `awk` command is not installed in the standard env. So this command fails if the `awk` command is not installed by some external module.

nixos/modules/installer/cd-dvd/sd-image.nix

@@ -223,7 +223,7 @@ in
         # Figure out device names for the boot device and root filesystem.
         rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
         bootDevice=$(lsblk -npo PKNAME $rootPart)
-        partNum=$(lsblk -npo MAJ:MIN $rootPart | awk -F: '{print $2}')
+        partNum=$(lsblk -npo MAJ:MIN $rootPart | ${pkgs.gawk}/bin/awk -F: '{print $2}')
 
         # Resize the root partition and the filesystem to fit the disk
         echo ",+," | sfdisk -N$partNum --no-reread $bootDevice