XDG_DATA_DIRS not set on non-nixos linux installation

[marius851000]

Describe the bug
The environment variable XDG_DATA_DIRS is not set with the file I put in .profile, resulting in app not appearing on the list of app and missing icon (I manage my icon with nix)

To Reproduce

1. Make a clean install of nix on non-nixos linux (used ubuntu with kubuntu-desktop)
2. Be sure to add what the installer asked in the .profile
3. install an app, for exemple firefox and uninstall the default installed one if any.
4. Disconnect/reconnect. The icon is missing in the app launcher, but it can be launched from the command line.

Expected behavior
Have the icon appear on the desktop. It can be done by adding export XDG_DATA_DIRS="$NIX_LINK/share:$XDG_DATA_DIRS" to the .profile file.

Metadata
Can't. Currently behind a proxy, and nix doesn't work when behind a proxy and proxy variable are set on a non-super user variable. Nix is not configured for the super user (this is tempory situation time I figure why nixos don't work with my screen)

[matthewbauer]

This is intentional I think:

NixOS/nix#2443 (comment)

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