netbird: fix GUI application's XDG desktop file

The PRs https://github.com/NixOS/nixpkgs/pull/496370 and
https://github.com/NixOS/nixpkgs/pull/499273 updated the netbird-ui
icon location in and removed Exec path from the netbird-ui wrapper,
respectively. However, this resulted in a bug where the both the Icon
and Exec paths pointed to non-existent files.

To address the regression, this PR adjusts the desktop file for the
netbird-ui wrapper to use the correct paths.

nixos/modules/services/networking/netbird.nix

@@ -340,7 +340,8 @@ in
                         substitute ${cfg.ui.package}/share/applications/netbird.desktop \
                             "$out/share/applications/${mkBin "netbird"}.desktop" \
                           --replace-fail 'Name=Netbird' "Name=NetBird @ ${client.service.name}" \
-                          --replace-fail 'Icon=netbird' "Icon=${cfg.ui.package}/share/pixmaps/netbird.png"
+                          --replace-fail 'Icon=netbird' "Icon=${cfg.ui.package}/share/icons/hicolor/256x256/apps/netbird.png" \
+                          --replace-fail 'Exec=netbird-ui' "Exec=${mkBin "netbird-ui"}"
                       '')
                     ];
                   };