thread-sanitizer reports data races here when running test-link. I tried
only removing the ones that triggered races, but after 10 back and
forths with the compiler and tsan, I got impatient and removed all of
them.

next time, let's be sure the test suite runs tsan-clean before merging
any changes that add parallelism.

after this commit, `zig build test-link` completes without any tsan
warnings.

closes #21778