nixos: Mount binds after the persistent storage path

[olmokramer]

This sets the depends (introduced in https://github.com/NixOS/nixpkgs/pull/86967) of each bind mount so that it will only be mounted after the persistent storage path has been mounted, as suggested in https://github.com/nix-community/impermanence/issues/22#issuecomment-847879359.

An obvious issue with this is that this will break older NixOS where the option does not exist yet. Any suggestions on how to work around that?

[cole-h]

If GitHub's tags are to be believed, that PR is present in both 21.05 and 21.11, and support for any version of NixOS older than those have expired. IME, this is fine; however, I'd want to document this new version bound. Something like "as of PR #66 (with a link), NixOS 21.05 or later is required for proper functionality", or however you'd care to word it with a similar effect.

Though, this is also just my opinion. cc @talyz @lovesegfault @etu

[olmokramer]

Ah I did not know that the patch was backported to all supported versions of NixOS. I agree that the change in version requirements should be documented as @cole-h suggests. I'll try to find some time in the weekend to make the changes.

[talyz]

Fixed in #115

[olmokramer]

Oh hey I totally forgot about this one. Thanks for picking it up again @Kranzes!