Fix "resizing" wrong partition when having two disks

[nevesenin]

Hi!

I discovered that the postBootCommands script didn't work under certain circumstances. I created an image with the following filesystem setup:

...

    fileSystems = {
      "/boot/firmware" = {
        label = config.raspberry-pi-nix.firmware-partition-label;
        device = "/dev/disk/by-label/${config.raspberry-pi-nix.firmware-partition-label}";
        fsType = "vfat";
      };
      "/" = {
        label = "NIXOS_SD";
        device = "/dev/disk/by-label/NIXOS_SD";
        fsType = "ext4";
      };
      "/foo/bar" = {
        label = "FOO_BAR";
        device = "/dev/disk/by-label/FOO_BAR";
        fsType = "ext4";
      };
    };

...

On the Pi I use a dual NVME HAT. One disk was flashed with the disk image and the other disk was previously prepared with an ext4 FS and labeled with FOO_BAR.
After booting the Pi I checked the devices with lsblk and found out that the disk I flashed with the created image was nvme1n1 instead of nvme0n1 and the additional disk was nvme0n1. lsblk output was something like this (from memory):

NAME                                          MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
nvme0n1                                       259:0    0 953.9G  0 disk  
└─nvme0n1p1                                   259:1    0 953.9G  0 part  /foo/bar
nvme1n1                                       259:2    0 238.5G  0 disk
├─nvme1n1p1                                   259:3    0   128M  0 part  /boot/firmware
├─nvme1n1p2                                   259:4    0   3.1G  0 part  /nix/store
|                                                                        /
├─nvme1n1p3                                   259:5    0     7M  0 part
└─nvme1n1p4                                   259:6    0   234G  0 part

The postBootCommands script created partition 4 (and 3) because it takes MAJ:MIN column to determine partition number, which is 4 in this case, but should be 2.
This happens when another disk is indexed before the flashed one.
After changing the script by using PARTN column of lsblk output, it looks as expected:

NAME                                          MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
nvme0n1                                       259:0    0 953.9G  0 disk  
└─nvme0n1p1                                   259:1    0 953.9G  0 part  /foo/bar
nvme1n1                                       259:2    0 238.5G  0 disk
├─nvme1n1p1                                   259:3    0   128M  0 part  /boot/firmware
└─nvme1n1p2                                   259:4    0 238.3G  0 part  /nix/store
                                                                         /