fix a pidpressure test flake

With the new busybox, ash has a built-in sleep command. Prior to this
change we were creating half the pids expected since `sleep` wasn't
actually launching a new binary.  Use the full path to /bin/sleep which
avoids the built-in and actually launches a new process.

test/e2e_node/eviction_test.go

@@ -982,7 +982,7 @@ func diskConsumingPod(name string, diskConsumedMB int, volumeSource *v1.VolumeSo
 
 func pidConsumingPod(name string, numProcesses int) *v1.Pod {
 	// Each iteration forks once, but creates two processes
-	return podWithCommand(nil, v1.ResourceRequirements{}, numProcesses/2, name, "(while true; do sleep 5; done)&")
+	return podWithCommand(nil, v1.ResourceRequirements{}, numProcesses/2, name, "(while true; do /bin/sleep 5; done)&")
 }
 
 // podWithCommand returns a pod with the provided volumeSource and resourceRequirements.