ci: create reusable workflow for nix kernel builds #6393

Job	Run time
lint	11s
build-kernel	14s
pages	0s
build-kernel-nix / build-kernel	4s
rust-test-schedulers (scx_bpfland)	10m 14s
layered-matrix (scx_layered, --disable-topology=false)	4m 10s
rust-test-core (scx_loader)	5m 48s
rust-test-schedulers (scx_lavd)	10m 16s
layered-matrix (scx_layered, --disable-topology=false, --disable-antistall)	3m 16s
integration-test (scx_bpfland)	4m 2s
rust-test-core (scx_rustland_core)	8m 26s
rust-test-schedulers (scx_layered)	10m 0s
layered-matrix (scx_layered, --disable-topology=true)	3m 25s
integration-test (scx_lavd)	4m 8s
rust-test-core (scx_stats)	3m 26s
rust-test-schedulers (scx_rlfifo)	11m 3s
integration-test (scx_layered)	4m 29s
layered-matrix (scx_layered, --disable-topology=true, --disable-antistall)	3m 11s
rust-test-core (scx_utils)	9m 36s
rust-test-schedulers (scx_rustland)	11m 4s
integration-test (scx_rlfifo)	4m 21s
rust-test-schedulers (scx_rusty)	9m 3s
integration-test (scx_rustland)	3m 38s
integration-test (scx_rusty)	4m 1s
	2h 8m 6s

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