dockerfile: update args definitions to llb.EnvList #4863

Job	Run time
build (windows/amd64)	1m 45s
build (freebsd/amd64)	1m 51s
test-freebsd-amd64	2m 18s
test-windows-amd64 (containerd, ./client)	4m 16s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 58s
test-windows-amd64 (containerd, ./worker/containerd)	3m 25s
test-windows-amd64 (containerd, ./solver)	3m 34s
test-windows-amd64 (containerd, ./frontend)	3m 1s
test-windows-amd64 (containerd, ./frontend/dockerfile)	31m 40s
test-windows-amd64 (./..., 1)	11m 30s
	1h 6m 18s

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