Add a test for reproducible builds

We should test if our builds are bytewise reproducible with the default flags.

An example of such a test is [in rules_r](https://github.com/grailbio/rules_r/blob/master/tests/repro/repro_test.sh).

I think because we are not setting `ZERO_AR_DATE=1` on macOS, at least our mac builds are not reproducible.

Start by reviewing the repository's existing build and test entry points, then compare the linked rules_r reproducibility test with this project's default build flags. Check the macOS path and ZERO_AR_DATE=1 behavior; done means a test verifies bytewise-reproducible builds with the default flags.

Written by the indexing model from the issue text.