make the /nix/store writable under netboot images

[cleverca22]

Motivation for this change

prior to this change, the netboot images mounted /nix/store read-only, and the nix-store --check-validity call in nixos-install tries to create /nix/store/.links on the host, causing it to fail

the mount config was simply copied from the iso-image.nix module, and tweaked to find the squashfs in the initrd

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• 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/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@cleverca22, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nshalman to be a potential reviewer.

[nshalman]

Certainly looks more elegant than my original code. :)

[dasJ]

@cleverca22 Seems like some necrobumping commenting here…
Is there any particualar reason you chose a fuse filesystem for overlaying? Seems like a slow solution compared to e.g. overlayfs

[cleverca22]

dont remember exactly, but i was probably copying another similar boot process in nixos, that already used fuse