Flatpak: Capture GCancellable objects with a strong ref
This is an alternative implementation of !789 (merged) in which g_object_ref/g_autoptr are not being duplicated across lambdas bodies, which means less codegen on that part, but it increases number or arguments in order to pass down the object via function calls which also generates some visual noise.
QCoro::Task coroutines start eagerly, so we don't need to ref it outside of a lambda, and g_autoptr will take care of cleaning it up regardless or how or when the coroutine completes.
Amends cc38c6c3