race condition for version migrations

[beckermr]

If a user opens their own version update PR in between the time the time the bot finds the new version, but has not yet updated its metadata on each feedstock to their current versions, then we get a race condition where the bot fails.

This happened for fa2_modified in a recent run: https://github.com/regro/cf-scripts/actions/runs/9304288652/job/25608500238

We may want to reparse the feedstock version from upstream one more time before we try and make a version PR. We'll still be subject to the race condition, but the time window for error will be a lot smaller (~few minutes instead of maybe ~1 hour).