roll: recipes_version.txt multi_roller: Upload changes together

Subrolls of multi rollers might upload changes at very different times.
This can cause issues because their presubmits might depend on changes
being in topics together and being uploaded at approximately the same
time. Change individual rollers to set a 'ready_to_upload' output
property when they're ready to upload, but then have them block until
the parent build sets the 'continue_to_upload' output property, which it
sets once it sees 'ready_to_upload' on all subrolls.

Original-Bug: b/478923716
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/381053
Original-Revision: 8c75a2f4361571bb3189d17ad0a019b192f0bba3
GitOrigin-RevId: 215d20e0472ddf13656cbf58ff8b6f7cd5d27068
Change-Id: Ie98580852683479b4475af704fdd18d9166564c1