Update keep_checkpoints_num to 100 million for all to ensure all checkpoints are saved

1 job for flatland-paper-baselines in 2 minutes and 28 seconds (queued for 38 seconds)
Name Stage Failure
test Test
  Downloading tensorflow-2.1.0-cp37-cp37m-manylinux2010_x86_64.whl (421.8 MB)

Pip subprocess error:
/tmp/tmpoSezCc: line 3: 317 Killed /opt/conda/envs/flatland-baseline-cpu-env/bin/python -m pip install -U -r /builds/flatland/neurips2020-flatland-baselines/tmpe_MXD3.requirements.txt

CondaEnvException: Pip failed

ERROR: Job failed: command terminated with exit code 1