cache: convert prune loop from recursive invocation to iterative #5183

Job	Run time
prepare	1s
test / prepare	2m 5s
test / run (containerd, ./frontend/dockerfile, integration)	5m 6s
test / run (containerd, ./frontend/dockerfile, dockerfile)	13m 52s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 7s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	7m 53s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	6m 8s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	13m 53s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 18s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 56s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 45s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	8m 1s
test / run (oci, ./frontend/dockerfile, integration)	5m 17s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 12s
test / run (oci-rootless, ./frontend/dockerfile, integration)	5m 1s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	7m 13s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 29s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 30s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 46s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 24s
image (mainline)	5m 45s
image (labs)	5m 29s
release	0s
scout	0s
	2h 18m 11s

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