nix/keys: Fix eval if storeKeysOnMachine is true.

This currently only fixes evaluation but actually doesn't apply the
correct permissions on the key files because they're just links to world
readable files in the Nix store.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nix/keys: Fix description of storeKeysOnMachine.

As storeKeysOnMachine no longer is true by default (7ae4b27), we
shouldn't say so in the description. Especially because the default
value is already shown in the generated manual along with the
description.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>