sdImage: use findmnt to get root device when resizing

[illegalprime]

Motivation for this change

when testing on an ARM board, /dev/disks/by-label wasn't available during this time in the boot process, (maybe I'm doing something wrong, but it did become available later during boot). this uses findmnt which seems more robust. it also specifies that the script exits on error (therefore trying again on the next boot) and logs commands to the console for clarity.

I've tested it and / is successfully resized on first boot.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[illegalprime]

bump
cc @dezgeg @infinisil

[infinisil]

The /dev/disk symlinks indeed are only available once udev is started which could take a bit of time. This sounds like a reasonable change, I'll merge.