ci: Simplify downloading kernel for crosvm

Directly download the kernel instead of using the
download-prebuilt-kernel.sh script.
Save the kernel to /kernel for clarity, replacing the previous
/lava-files directory.

Signed-off-by: Valentine Burley <valentine.burley@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33606>

.gitlab-ci/common/init-stage2.sh

@@ -92,9 +92,9 @@ if [ "$HWCI_KVM" = "true" ]; then
     } || \
         modprobe ${KVM_KERNEL_MODULE}
 
-    mkdir -p /lava-files
+    mkdir -p /kernel
     curl -L --retry 4 -f --retry-all-errors --retry-delay 60 \
-	-o "/lava-files/${KERNEL_IMAGE_NAME}" \
+	-o "/kernel/${KERNEL_IMAGE_NAME}" \
         "${KERNEL_IMAGE_BASE}/amd64/${KERNEL_IMAGE_NAME}"
 fi