record script seeded time before seeding scripts to reduce race conditions

It seems like we've had a few instances of staging being in a bad state because files were changed while the seeding is going on. Trying to set the mtime earlier so that we err more on the side of reseeding things that we have already seeded (which should be fine, given this is what happens all the time in other envs) rather than not seeding things that actually need to be.