improve alpine version selection per arch #2703

Job	Run time
prepare	1s
test / prepare	2m 12s
test / run (containerd, ./frontend/dockerfile, integration)	4m 39s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 38s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 28s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 42s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 36s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	5m 50s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 42s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 7s
test / run (oci, ./frontend/dockerfile, integration)	4m 19s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 6s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 59s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	4m 58s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 26s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	4m 57s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 20s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 12s
image (mainline)	4m 49s
image (labs)	5m 1s
release	0s
	1h 32m 2s

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