src: use correct microtask queue for checkpoints
I missed in c6c83374 that we should not just use that queue for enqueuing microtasks, but also for running them.
Refs: https://github.com/nodejs/node/pull/36482
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines