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.
Prior to this change GTK_USE_PORTAL was unconditionally
set to "1". For this to not break things you have to have some
sort of portal implementation in extraPortals.
Setting GTK_USE_PORTAL in this manner is actually only useful
when using portals for applications outside flatpak. For example
people using non-flatpak Firefox who want native filechoosers.
It's also WIP for electron applications to support this.
Left to do: re-enable as needed in the usual situations.
This added ~286MiB to the base system closure, which is enough to bring
the sd images over the limit allowed on Hydra.
