nixos/gdm: Fix missing icons
nixos/gdm: Fix missing icons

Recent `wrapGAppsHook` change stops `adwaita-icon-theme` from being added to `XDG_DATA_DIRS`:
https://github.com/NixOS/nixpkgs/commit/b1e73fa2e086f1033a33d93524ae2a1781d12b95
Since `display-manager.service` does not have `/run/current-system/sw/share` in `XDG_DATA_DIRS`, it does not pick up the globally installed icon theme either, preventing icons from showing.

Let’s make Adwaita available to fix that for now.

Fixes: https://github.com/NixOS/nixpkgs/issues/171692

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