ci: get bpf clang from pinned nixpkgs #3754

Job	Run time
lint	26s
build-kernel	20s
pages	0s
layered-matrix (scx_layered, --disable-topology=false)	4m 33s
layered-matrix (scx_layered, --disable-topology=false, --disable-antistall)	4m 0s
integration-test (scx_bpfland)	4m 50s
layered-matrix (scx_layered, --disable-topology=true)	3m 59s
integration-test (scx_lavd)	5m 1s
rust-test-core (scx_loader)	4m 30s
layered-matrix (scx_layered, --disable-topology=true, --disable-antistall)	4m 39s
rust-test-schedulers (scx_bpfland)	5m 45s
integration-test (scx_layered)	6m 7s
rust-test-core (scx_rustland_core)	5m 42s
integration-test (scx_rlfifo)	3m 55s
rust-test-core (scx_stats)	3m 42s
rust-test-schedulers (scx_lavd)	7m 28s
rust-test-core (scx_utils)	5m 32s
integration-test (scx_rustland)	4m 35s
rust-test-schedulers (scx_layered)	6m 32s
integration-test (scx_rusty)	5m 43s
rust-test-schedulers (scx_rlfifo)	6m 11s
rust-test-schedulers (scx_rustland)	6m 52s
rust-test-schedulers (scx_rusty)	6m 16s
	1h 46m 38s

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