chore: tidy up removal of digest algorithm #2235

Job	Run time
prepare	4s
binaries (darwin/amd64)	1m 5s
binaries (darwin/arm64)	1m 1s
binaries (linux/amd64)	2m 3s
binaries (linux/arm/v7)	2m 15s
binaries (linux/arm64)	2m 8s
binaries (linux/s390x)	2m 2s
binaries (linux/ppc64le)	2m 2s
binaries (linux/riscv64)	2m 4s
binaries (windows/amd64)	1m 9s
binaries (windows/arm64)	1m 1s
test / prepare	1m 40s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 43s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 31s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	6m 32s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 21s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 7s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 6s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 50s
test / run (./..., 1, integration gateway)	3m 58s
test / run (./client, containerd, nydus, integration)	6m 39s
test / run (./client, oci, nydus, integration)	4m 58s
test / run (./..., nydus, 1, integration)	3m 49s
image	7m 20s
image (rootless)	7m 33s
release	10s
	1h 40m 11s

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