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_metrics.sh

@@ -80,8 +80,8 @@ if [ "${TEST_ARCH}" == "aarch64" ]; then
     guestunmount "$FOCAL_OS_RAW_IMAGE_UPDATE_TOOL_ROOT_DIR"
 fi
 
-# Build custom kernel based on virtio-pmem and virtio-fs upstream patches
-build_custom_linux
+# Download prebuild linux binaries
+download_linux
 
 CFLAGS=""
 if [[ "${BUILD_TARGET}" == "${TEST_ARCH}-unknown-linux-musl" ]]; then