... via _NIX_TEST_NO_ENVIRONMENT_WARNINGS
This environment variable suppresses warnings that depend on the test
environment (such as ulimit warnings in builds on systems with lower
limits, which may well succeed if it weren't for the warning).
This prevents non-deterministic test failures in golden/characterization
tests.
Alternative considered: filtering stderr in test scripts, but that approach
is fragile with binary test output, and potentially multiple call sites.