[ci][buildkite] Fix nix blob tests pipeline

Summary:
Now when blob service is in Rust, this workflow is broken. Hopefully this job can simply just work again.

I did the same steps as in identity_tests.yml.

Follow ups

Now I put these steps directly in Buildkite. After this is landed, we can point it back to the yml file. And perhaps enable this in Harbormaster.

Test Plan: Test build passes: https://buildkite.com/comm/blob-unit-tests-nix/builds/58#0184ce7a-4596-4500-9bfb-cf0888095e35

Reviewers: atul, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5789