xdg-portal: assert that required paths are linked on NixOS

[bmrips]

Description

The description of xdg.portal.enable that certain paths need to be added to NixOS' environment.pathsToLink option when Home Manager is installed as a submodule and home-manager.useUserPackages is enabled. This PR adds precisely this check as an assertion.

Checklist

• Change is backwards compatible.

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[bmrips]

I just found out that, instead of pointing out the problem, we can also fix it automatically, as I implemented here. However, I am not sure whether this is modification to the NixOS configuration might be too 'invasive'. @khaneliman can you give me some advice on this matter?

[khaneliman]

I just found out that, instead of pointing out the problem, we can also fix it automatically, as I implemented here. However, I am not sure whether this is modification to the NixOS configuration might be too 'invasive'. @khaneliman can you give me some advice on this matter?

Yeah, I generally prefer a warning/assertion over magically enabling something. But, we can also offer an integration option or a toggle that allows someone to get the extra configuration by "allowing" or "opting in" to the change.

[bmrips]

Like so? 200599c

[khaneliman]

Like so? 200599c

I'm having second thoughts lol. just wondering if it blurs the line between nixos module and home-manager responsibilities.

This would then open the door for other modules to just add their own NixOS configurations to the nixos module and I don't know if I want to set that precedent...

@rycee @ambroisie any thoughts?