third_party/linux: build using unhermetic rule

This replaces ad-hoc genrules (for the node Linux image and the ktest
image) with a real Bazel rule with an attached transition which ensures
we end up with the same-ish configurations for all builds of an image.

This reduces rebuilds of the ktest Linux kernel, from three down to one.

Before: https://drive.google.com/file/d/1c6VmY2bqx9Pgs61TOUfgMi8Sn0WQeobu/view

After: https://drive.google.com/file/d/13eO1rLhoBCMMRUKrmJz8QnhdAR3ctIGb/view

We also drive-by fix the Kubernetes CTS test suite to run on a single-node
Cluster (instead of failing early due to that being currently reworked).

Test Plan: Build system refactor, following existing test.

X-Origin-Diff: phab/D761
GitOrigin-RevId: b5545ac5fd402fbf0340d941a90b9ea6ea0b6d43
diff --git a/metropolis/test/launch/launch.go b/metropolis/test/launch/launch.go
index 3a444ef..85e0a21 100644
--- a/metropolis/test/launch/launch.go
+++ b/metropolis/test/launch/launch.go
@@ -521,7 +521,7 @@
 
 	go func() {
 		if err := RunMicroVM(ctx, &MicroVMOptions{
-			KernelPath:             "metropolis/test/ktest/linux-testing.elf",
+			KernelPath:             "metropolis/test/ktest/vmlinux",
 			InitramfsPath:          "metropolis/test/nanoswitch/initramfs.lz4",
 			ExtraNetworkInterfaces: switchPorts,
 			PortMap:                portMap,