[avalanche] Rename variables shadowing builtins in avatools

Summary: Shadowing builtins can lead to unexpected behavior. This is one of many steps to integrating flake8-builtins so we do not have to worry about this in the future.

Test Plan:

./test/functional/test_runner.py abc_*
pip3 install flake8-builtins
flake8 test/functional/test_framework/avatools.py  # verify no A00x errors

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13660