llb: use buildkit user-agent for HTTP source #5090

Job	Run time
build (windows/amd64)	1m 40s
build (freebsd/amd64)	1m 36s
test-freebsd-amd64	2m 42s
test-windows-amd64 (containerd, ./client)	3m 35s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 21s
test-windows-amd64 (containerd, ./worker/containerd)	2m 49s
test-windows-amd64 (containerd, ./solver)	2m 32s
test-windows-amd64 (containerd, ./frontend)	2m 35s
test-windows-amd64 (containerd, ./frontend/dockerfile)	29m 39s
test-windows-amd64 (./..., 1)	10m 53s
	1h 0m 22s

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