Fix vm test not respecting disko.tests.extraConfig disko overwrites

[BerriJ]

@Lassulus This should fix cases where users overwrite some disko configurations inside of disko.tests.extraConfig. The docs on this setting state:

Extra NixOS config for your test. Can be used to specify a different luks key for tests.
A dummy key is in /tmp/secret.key

Consider the following setting: You have boot.initrd.systemd.enable = true; (systemd in Stage 1) and you have a disko config with luks like this:

type = "luks";
name = "main";
extraOpenArgs = [ "--allow-discards" ];
passwordFile = lib.mkDefault "/tmp/secret.key";
askPassword = false;
settings = {
  allowDiscards = true;
};

So usually you provide a passwordFile and when booting you just type that password.

However, for VM tests you want to unlock this luks partition non-interactively. So you:

• Create a systemd service that provides /tmp/secret.key during boot
• Set settings.keyFile = "/tmp/secret.key so that cryptsetup uses the keyFile to unlock the partition

So I expected this to work:

disko.tests.extraConfig = {
  disko.devices.disk.main.content.partitions.main.content.settings.keyFile = "/tmp/secret.key";
};

While nix repl does show:

❯ nix repl .
Nix 2.31.2+1
Type :? for help.
warning: Git tree '/home/jonathan/git/DSEE/NIXOS/disko-reprex' is dirty
Loading installable 'git+file:///home/jonathan/git/DSEE/NIXOS/disko-reprex#'...
Added 1 variables.
nixosConfigurations
nix-repl> nixosConfigurations.test.config.virtualisation.vmVariantWithDisko.boot.initrd.luks.devices.main.keyFile
"/tmp/secret.key"

Running

❯ nix run github:nix-community/nixos-anywhere -- \
  --flake .#test \
  --vm-test

Will stop at the interactive password prompt.

With this PR, it will respect the keyFile setting correctly and auto-unlock.

See #1185 for a reproducible example.

[Enzime]

This looks like the same changes I made in #480 however I think the correct approach is #1094 (comment)