[build] avbtool -> avbtool.py

Note we have to retain the original avbtool for a soft transition.
Will delete it in a follow-up.

Bug: 64359
Change-Id: I86d8fe0d161051edc917aa7184a2f4982ceccab8
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/450606
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Testability-Review: Shai Barack <shayba@google.com>
Fuchsia-Auto-Submit: Shai Barack <shayba@google.com>