flatpak-utils-http: Don’t use GMainLoop
flatpak-utils-http: Don’t use GMainLoop

Iterate the `GMainContext` manually instead. This makes the exit
condition from the loop much more obvious, and eliminates the potential
class of problems where `g_main_loop_quit()` is called before
`g_main_loop_run()` and has no effect.

Signed-off-by: Philip Withnall <12d956b70be7f0d60e54e48e97a01dac38dce989@endlessos.org>

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