Creating a new sorted list of sstate tasks each iteration through runqueue is
extremely ineffecient and was compounded by the recent change from a list to set.
Create one sorted list instead of recreating it each time.
(Bitbake rev: de18824996841c3f35f54ff5ad12f94f6dc20d88)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>