add support for AlwaysReplaceExistingDestPaths in llb copy #2780

Job	Run time
prepare	1s
test / prepare	1m 59s
test / run (containerd, ./frontend/dockerfile, integration)	5m 31s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 27s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 33s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 59s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 45s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	6m 18s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 16s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 41s
test / run (oci, ./frontend/dockerfile, integration)	4m 20s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 22s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 11s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 11s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 13s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 5s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 32s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 26s
image (mainline)	4m 54s
image (labs)	5m 14s
release	0s
	1h 36m 58s

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