nixos: Fix eval error for documentation.nixos

Introduced by 0f3b89bb

.

If services.nixosManual.showManual is enabled and
documentation.nixos.enable is not, there is no
config.system.build.manual available, so evaluation fails. For example
this is the case for the installer tests.

There is however an assertion which should catch exactly this, but it
isn't thrown because the usage of config.system.build.manual is
evaluated earlier than the assertions.

So I split the assertion off into a separate mkIf to make sure it is
shown appropriately and also fixed the installation-device profile to
enable documentation.nixos.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @oxij