Save build failure logs to S3

[fredemmott]

Using the existing bucket for now; will make another one with custom retention policies if we can't fix the flakiness quickly.

Test Plan:

Shellcheck.

Land. Tomorrow:

• check for corresponding success/failure docker images
• check for log files in s3 if there are failures
• revert if there's problems, so we get good signal over the holidays

[fredemmott]

This is hopefully temporary while figuring out why things aren't being published to where bin/list-build-logs reads from