build-tracker: always use the commit author

Created by: burmudar

When a build is retried on buildkite it gets a new build number and it's author gets zeroed out. Since the build has a new build number, build-tracker tracked it as a new build trusting that it has it's author set. This lead to something like the following:

Varun retried his build twice, his first build has his name set, but subsequent builds didn't

1. Build one
2. Build Two
3. Build Three

To get a teammates information we used the build commit and then resolved that in github to figure out who the person was. Unfortunately, we checked the build to make sure we have an author, even though the build author isn't used AT ALL anymore! Hence the fix :D

Test plan

Unit tests!