nixos/xdg/portal: set GTK_USE_PORTAL with lib.mkIf

If lib.optional is given a false value it will return an empty list.
Thusly the set-environment script can have

```
export GTK_USE_PORTAL=
```

This can rub certain bugs the wrong way #65679
so lets make sure this isn't set in the environment
at all.

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/config/xdg/portal.nix b/nixos/modules/config/xdg/portal.nix
index 89ddf80b575..bdbbfda2bb4 100644
--- a/nixos/modules/config/xdg/portal.nix
+++ b/nixos/modules/config/xdg/portal.nix
@@ -51,7 +51,7 @@ with lib;
       systemd.packages = packages;
 
       environment.variables = {
-        GTK_USE_PORTAL = optional cfg.gtkUsePortal "1";
+        GTK_USE_PORTAL = mkIf cfg.gtkUsePortal "1";
         XDG_DESKTOP_PORTAL_PATH = map (p: "${p}/share/xdg-desktop-portal/portals") cfg.extraPortals;
       };
     };