remotecache: fix missing CheckDescriptor method #2679

Job	Run time
prepare	1s
test / prepare	5m 19s
test / run (containerd, ./frontend/dockerfile, integration)	4m 38s
test / run (containerd, ./frontend/dockerfile, dockerfile)	5m 54s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 54s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 52s
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)	5m 36s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 35s
test / run (oci, ./frontend/dockerfile, integration)	4m 10s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 45s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 43s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 23s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 15s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 35s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 21s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 52s
image (mainline)	4m 42s
image (labs)	4m 48s
release	0s
	1h 38m 49s

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