refactor(@angular/build): update `ensureWorkerPool` to do an early exit #1187

Job	Run time
list	30s
workflow (build-cli)	1m 42s
workflow (modules-builder-tests)	1m 52s
workflow (build-unit-tests)	2m 25s
workflow (devkit-core-tests)	1m 25s
workflow (devkit-core-tests-rerun)	1m 31s
	9m 25s

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