ci: create a reusable action for loading kernel cache #6400

Job	Run time
build-kernel-nix	4s
build-kernel	15s
lint	12s
pages	0s
layered-matrix (scx_layered, --disable-topology=false)	3m 11s
rust-test-core (scx_loader)	5m 17s
integration-test (scx_bpfland)	3m 57s
rust-test-core (scx_rustland_core)	7m 4s
layered-matrix (scx_layered, --disable-topology=false, --disable-antistall)	3m 53s
rust-test-schedulers (scx_bpfland)	2m 19s
rust-test-core (scx_stats)	3m 29s
layered-matrix (scx_layered, --disable-topology=true)	3m 50s
integration-test (scx_lavd)	4m 4s
rust-test-schedulers (scx_lavd)	2m 13s
rust-test-core (scx_utils)	6m 20s
layered-matrix (scx_layered, --disable-topology=true, --disable-antistall)	3m 16s
integration-test (scx_layered)	4m 9s
rust-test-schedulers (scx_layered)	2m 5s
integration-test (scx_rlfifo)	3m 21s
rust-test-schedulers (scx_rlfifo)	2m 2s
integration-test (scx_rustland)	3m 41s
rust-test-schedulers (scx_rustland)	1m 50s
integration-test (scx_rusty)	4m 26s
rust-test-schedulers (scx_rusty)	1m 50s
	1h 12m 48s

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