build: Use binary kernel artifacts

The Linux kernel fork repository for Cloud Hypervisor now produces
prebuilt x86-64 and aarch64 binaries. Speed up the CI by using those
binaries.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

scripts/run_integration_tests_live_migration.sh

@@ -56,7 +56,8 @@ popd || exit
 # Build custom kernel based on virtio-pmem and virtio-fs upstream patches
 VMLINUX_IMAGE="$WORKLOADS_DIR/vmlinux"
 if [ ! -f "$VMLINUX_IMAGE" ]; then
-    build_custom_linux
+    # Download prebuild linux binaries
+    download_linux
 fi
 
 CFLAGS=""