rewrite-timestamp: fix incompatibility with `COPY --link` #3136

Job	Run time
build (windows/amd64)	1m 47s
build (freebsd/amd64)	1m 34s
test-windows-amd64 (containerd, ./client)	5m 5s
test-freebsd-amd64	16m 20s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 7s
test-windows-amd64 (containerd, ./worker/containerd)	3m 46s
test-windows-amd64 (containerd, ./solver)	3m 1s
test-windows-amd64 (containerd, ./frontend)	2m 16s
test-windows-amd64 (containerd, ./frontend/dockerfile)	5m 45s
test-windows-amd64 (./..., 1)	13m 30s
	56m 11s

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