boot.postBootCommands should be synced with upstream

[DerRockWolf]

Currently, if setting sdImage.expandOnBoot = false; the resulting NixOS installation isn't fully functional because the whole postBootCommands script is skipped.

raspberry-pi-nix/sd-image/sd-image.nix

Lines 249 to 274 in d342e97

	boot.postBootCommands = lib.mkIf config.sdImage.expandOnBoot ''
	# On the first boot do some maintenance tasks
	if [ -f /nix-path-registration ]; then
	set -euo pipefail
	set -x
	# 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 | ${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
	${pkgs.parted}/bin/partprobe
	${pkgs.e2fsprogs}/bin/resize2fs $rootPart
	# Register the contents of the initial Nix store
	${config.nix.package.out}/bin/nix-store --load-db < /nix-path-registration
	# nixos-rebuild also requires a "system" profile and an /etc/NIXOS tag.
	touch /etc/NIXOS
	${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/system --set /run/current-system
	# Prevents this from running on later boots.
	rm -f /nix-path-registration
	fi
	'';

This has been fixed upstream at NixOS/nixpkgs#341071