fix: non-deterministic tests in export_upload_test

[Whoops]

Summary of changes

Asana Ticket: NO TICKET

[Please include a brief description of what was changed]

Reviewer Checklist

• Meets ticket's acceptance criteria
• Any new or changed functions have typespecs
• Tests were added for any new functionality (don't just rely on Codecov)
• This branch was deployed to the staging environment and is currently running with no unexpected increase in warnings, and no errors or crashes.

[Whoops]

My first theory was that with the tests all writing to the same directory, maybe two different tests were clobbering each other's data. This didn't fix the problem, but I left it in since it seemed like good practice.

[Whoops]

To replicate the failure locally, you can checkout master and run while mix test test/arrow/hastus/export_upload_test.exs; do :; done (assuming you're using bash or a bash-compatible shell). I'd estimate around 1 in 10 fail.

[Whoops]

Here's a CI run that had the failure.

[jzimbel-mbta]

Looks good, just leaving a "comment" review for now to get github slackbot to stop reminding about this PR until we get some input from Shanti on #1271 next week in two weeks.