Instead of simply adding 5 to the parent block time.

We really do need a deterministic time for the migration block so that
all parties that run the migration arrive at the same migration block
but the problem is that op-geth requires that the L2 migration block
(aka l2 origin) occurs after the l1 origin (I guess the point where you
deploy the bridge contracts to the l1). When we migrate a partially
synced datadir the block before the transition block will be very old,
up to 4 years old! So of course it occurs before the l1 origin. So a fix
just to get things working is to use time.Now(), but probably we should
make this a configurable parameter.