macOS build deadlocks nondeterministically

[Vtec234]

As seen here for example. My conjecture there was that this is due to the mutex in the lakefile.lean, but reproducing it locally it seems that there is sometimes a stray npm run build -- --tsxName <blah> process that never exits. This would suggest that the issue is actually something with NPM or Rollup.js.

[digama0]

Could this be related to https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/apple.20runners.20keep.20timing.20out/near/411786801 ?

[Vtec234]

I can also reproduce it locally, so it is unlikely to be an issue with the runners.