Give more time to polkitd to be ready

For unknown reasons (race conditions?), pkaction randomly fails with:
"Error enumerating actions: GDBus.Error:org.freedesktop.DBus.Error.NoReply:
Message recipient disconnected from message bus without replying"

So, to mitigate this issue give it some more time to be ready,
although this should be the aim of "gdbus wait" above.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>