shell-app: Do not allocate memory during GSpawnChildSetupFunc

shell-app: Do not allocate memory during GSpawnChildSetupFunc

The child context setup function used as launch GSpawnChildSetupFunc
uses g_object_get that internally potentially allocates memory, making
it not async-signal-safe and so not something that is safe to use in
between fork and exec, so just use the simpler getter here.

Note that the current implementation of app_child_setup() is safe
already as sd_journal_stream_fd, dup2 and close are so.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3293>
(cherry picked from commit dcb5956dea9c46720feaa1773ca9488327e34f51)