ibusManager: pass on envrionment variables to ibus-daemon

In GNOME 42, the fallback ibus spawn method was changed from using Gio.SubprocessLauncher to GLib.spawn_async(), this resulted in two issues for me.

The first issue is that the environment variables don't get passed on to the child, in the GLib.spawn_async() docs, if envp is not null, then only what is specified for envp will get passed on to the child, this is a problem since ibus expects to also have WAYLAND_DISPLAY on wayland and DBUS_SESSION_BUS_ADDRESS for dbus, which results in ibus not working properly.

The second problem I ran into is that ibus immediately exits saying that the parent process died. According to a person on the matrix, that is because g_spawn_async() double forks and ibus is detecting the death of the temporary process.

I discussed this on the matrix and we weren't able to come up with a solution for the latter problem, so this at least fixes the first issue. This issue is now resolved.

Feedback would be appreciated.