make sure RPATH'd helper executables all turn off GIO modules
Side issue from https://github.com/ValveSoftware/steam-runtime/issues/604.
A NixOS user's log contains lots of GIO modules failing to load:
/nix/store/dc53arqhqjh32jpbnn5jdai5qqqgsbi9-gvfs-1.50.5/lib/gvfs/libgvfscommon.so: undefined symbol: g_task_set_source_tag
I suspect this means we're loading GIO modules into one of our executables that has a RPATH
to our bundled GLib, which is old. We should make sure all our RPATH'd executables turn those off.