Use higher priority for PVF preparation in dispute/approval context (#4172)

Related to https://github.com/paritytech/polkadot-sdk/issues/4126
discussion

Currently all preparations have same priority and this is not ideal in
all cases. This change should improve the finality time in the context
of on-demand parachains and when `ExecutorParams` are updated on-chain
and a rebuild of all artifacts is required. The desired effect is to
speed up approval and dispute PVF executions which require preparation
and delay backing executions which require preparation.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>