There's no need to use polkit in the history command, so don't start the
agent in flatpak-main.c. This means we can avoid a test failure in
test-history.sh which was caused by old versions of valgrind being
unaware of syscall sched_getattr, which is used in g_bus_get_sync(),
itself called by install_polkit_agent().
(cherry picked from commit dd9fe190661bce70adf4e3df972b8033a656d48a)
0a4491aed7