feat(desktop-entries): Nix application paths support

[mmstick]

The Nix system path takes priority over the system path, but below Flatpak and Snap system paths, and the local nix path takes priority over all system paths, but below local Flatpak/Snap/.local paths.

To install nix in a multi-user capacity (system-wide):

sh <(curl -L https://nixos.org/nix/install) --daemon

Then you can install an application per-user and see it installed at ~/.nix-profile/share/applications.

nix-env -i firefox

To install system-wide, switch to root and do the same and you'll find it installed in /nix/var/nix/profiles/default/share/applications

sudo su
bash
nix-env -i firefox

[jacobgkau]

The Nix system path takes priority over the system path, but below Flatpak and Snap system paths, and the local nix path takes priority over all system paths, but below local Flatpak/Snap/.local paths.

After I installed Firefox with Nix, I still see the Flatpak option below it in search results (which seems correct), but I don't see the system-installed Firefox in the results anymore:

Is this expected? I would think either I should see all of the Firefox versions (which I'd prefer), or else only one.

[mmstick]

It is expected behavior because the Flatpak has a differing desktop ID from the debian and nix versions of it. The Flatpak uses org.mozilla.firefox and both the debian and nix versions use firefox.