[monorepo roller] Fix loop counter to limit number of rolls to 100

Change-Id: I40188d650eb6e7cc978bb94b30f9024ccae743c7
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/343783
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: Jonas Termansen <sortie@google.com>