Add FileActionSymlink and `llb.Symlink` #4928

Job	Run time
prepare	1s
test / prepare	2m 1s
test / run (containerd, ./frontend/dockerfile, integration)	4m 56s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 17s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 49s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 22s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 52s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 19s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 0s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 22s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 23s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	9m 25s
test / run (oci, ./frontend/dockerfile, integration)	4m 59s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 8s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 17s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 59s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 36s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 13s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 37s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 57s
image (mainline)	5m 30s
image (labs)	5m 9s
scout	0s
release	0s
	1h 58m 12s

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