Describe the bug
A clear and concise description of what the bug is.

When you set boot.zfs.enableUnstable, zfsUnstable is used but zfsStable is still put into the system closure. This causes an abort when the kernel is marked as incompatible with zfsStable which is very confusing because it should only be using zfsUnstable.

This only happens on 20.09, not on master.

To Reproduce
Steps to reproduce the behavior:

1. Mark zfsStable as incompatible with your kernel version
2. Set boot.zfs.enableUnstable
3. Build system closure

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

#108944

Notify maintainers

@Mic92 @NeQuissimus

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: