fix riscv64 build #1641

Job	Run time
prepare	1s
prepare	1s
test / prepare	15s
test / prepare	8m 3s
test / run	0s
image	0s
release	0s
release	0s
test / run (containerd, ./frontend/dockerfile, integration)	6m 51s
test / run (containerd, ./frontend/dockerfile, dockerfile)	10m 57s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	6m 29s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	9m 21s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	9m 2s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	12m 22s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	9m 55s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	10m 22s
test / run (oci, ./frontend/dockerfile, integration)	7m 40s
test / run (oci, ./frontend/dockerfile, dockerfile)	8m 18s
test / run (oci-rootless, ./frontend/dockerfile, integration)	7m 18s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	7m 46s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	8m 29s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	10m 2s
image (mainline)	9m 55s
image (labs)	7m 41s
	2h 30m 48s

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