scx_layered: Fix idle core selection #585

Job	Run time
pages	0s
build-kernel	29s
lint	18s
rust-test-schedulers (scx_bpfland)	4m 44s
integration-test (scx_bpfland)	3m 12s
rust-test-schedulers (scx_lavd)	5m 36s
rust-test-core (scx_loader)	3m 23s
integration-test (scx_lavd)	4m 6s
rust-test-schedulers (scx_layered)	5m 31s
integration-test (scx_layered)	3m 20s
rust-test-core (scx_rustland_core)	4m 40s
rust-test-schedulers (scx_rlfifo)	5m 13s
integration-test (scx_rlfifo)	3m 22s
integration-test (scx_rustland)	3m 38s
rust-test-schedulers (scx_rustland)	6m 3s
rust-test-core (scx_stats)	2m 13s
integration-test (scx_rusty)	3m 46s
rust-test-schedulers (scx_rusty)	5m 12s
rust-test-core (scx_utils)	4m 34s
	1h 9m 20s

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