ci: add installation step for LLVM 21 in llvm-backend lane #8

Job	Run time
parse conformance	41s
runtime smoke	45s
SIMD contract (Cranelift)	33s
type + lowering conformance	40s
fmt + clippy	45s
backend features (default)	54s
backend features (llvm-backend)	1m 0s
	5m 18s

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