Update github.com/containerd/containerd to v1.5.7

... to include a fix for
https://github.com/advisories/GHSA-c2h3-6mxw-7mvq .

(Note that Skopeo doesn't depend on the vulnerable code,
so this is primarily to avoid dependency checker warnings.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/google.golang.org/grpc/picker_wrapper.go

@@ -147,7 +147,7 @@ func (pw *pickerWrapper) pick(ctx context.Context, failfast bool, info balancer.
 			logger.Error("subconn returned from pick is not *acBalancerWrapper")
 			continue
 		}
-		if t, ok := acw.getAddrConn().getReadyTransport(); ok {
+		if t := acw.getAddrConn().getReadyTransport(); t != nil {
 			if channelz.IsOn() {
 				return t, doneChannelzWrapper(acw, pickResult.Done), nil
 			}