Allow extra files in the tarball

[pedorich-n]

Problem

I would like to include extra files in the tarball without exposing them to the /nix/store (e.g. not using environment.etc or systemd.tmpfiles).

Use case: include custom SSH host keys.

Solution

I think the easiest thing would be to add a --extra-files flag to the tarballBuilder script, which is expected to be a folder with the correct directory structure inside, and then do cp -R "${extraFiles}/." "$root". Similar to how it's already done for the config:

NixOS-WSL/modules/build-tarball.nix

Lines 87 to 89 in dee4425

	mkdir -p "$root/etc/nixos"
	cp -R ${lib.cleanSource cfg.configPath}/. "$root/etc/nixos"
	chmod -R u+w "$root/etc/nixos"

Inspired by NixOS-anywhere's --extra-files: https://github.com/nix-community/nixos-anywhere/blob/97b45ac774699b1cfd267e98a8bdecb74bace593/docs/howtos/extra-files.md

[terlar]

There was some discussion on this topic inside #304