chore: temporarily disable riscv64 build #1616

Job	Run time
prepare	2s
test / prepare	2m 22s
test / run (containerd, ./frontend/dockerfile, integration)	9m 26s
test / run (containerd, ./frontend/dockerfile, dockerfile)	12m 59s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	6m 27s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	12m 1s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	8m 50s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	12m 39s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	7m 0s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	11m 24s
test / run (oci, ./frontend/dockerfile, integration)	5m 59s
test / run (oci, ./frontend/dockerfile, dockerfile)	10m 24s
test / run (oci-rootless, ./frontend/dockerfile, integration)	6m 17s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	12m 2s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	7m 37s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	11m 39s
image (mainline)	13m 5s
image (labs)	12m 32s
release	0s
	2h 42m 45s

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