cache: Fix walkBlob to call callback only for unlazied contents #5044

Job	Run time
prepare	1s
test / prepare	2m 5s
test / run (containerd, ./frontend/dockerfile, integration)	5m 9s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 7s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 15s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 46s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 0s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	6m 54s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	4m 57s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 17s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 0s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 29s
test / run (oci, ./frontend/dockerfile, integration)	4m 29s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 44s
test / run (oci-rootless, ./frontend/dockerfile, integration)	5m 17s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 46s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	5m 4s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 19s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 33s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 56s
image (mainline)	5m 12s
image (labs)	5m 24s
scout	0s
release	0s
	1h 58m 44s

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