flatpak: clear GDK_PIXBUF_MODULE_FILE
GDK_PIXBUF_MODULE_FILE is often set to a nix store path not available in a app
sandbox. This can cause some apps to fail launching, simply reset this env var
when running applications.
fixes https://github.com/NixOS/nixpkgs/issues/53441
(cherry picked from commit 752e176d67e41b6b1d2da20a0c9c70079fe56a60)
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