[blob] Switch Blob unit tests to autoscaling CI runners

Summary:
Switch Blob unit test Buildkite workflow to use autoscaling CI runners.
Context: We discussed this in the retro this morning and I already had a proof of concept for this that I jotted down here: https://linear.app/comm/issue/ENG-5770/office-mba-runner-out-of-storage#comment-d61c38e9

Not actually doing this task right now, just figure I'd diff up what I already had and tested.

Test Plan:
CI

Reviewers: ashoat, ginsu, tomek, rohan, bartek

Reviewed By: bartek

Differential Revision: https://phab.comm.dev/D10328