nixos: make GIO_EXTRA_MODULES a session variable

[ncfavier]

At the moment, Thunar doesn't render thumbnails for files in the trash, because the tumbler instance started by dbus can't find the gvfs GIO extra modules (tumbler doesn't depend on gvfs so they're not added to GIO_EXTRA_MODULES in the wrapGAppsHook wrapper, if I understand things correctly).

This PR makes GIO_EXTRA_MODULES a sessionVariable instead of a variable in the three modules that define it, including the gvfs module. This shouldn't break anything, and I have confirmed that it fixes my issue.

cc @worldofpeace @jtojnar

[jtojnar]

It might potentially cause conflicts if someone sets environment.variables.GIO_EXTRA_MODULES manually, see #122244. But not much we can do about that.

[ncfavier]

Ah yeah I just noticed that. But that's a separate issue that should be resolved eventually.

[jtojnar]

Thanks, let’s give it a spin.