Explicitly provide kernel-root argument to prepare-rootfs action

Let's make the "kernel-root" explicit when using the prepare-rootfs
action, instead of relying on the default, .kernel.

Signed-off-by: Daniel Müller <deso@posteo.net>

.github/actions/vmtest/action.yml

@@ -78,9 +78,10 @@ runs:
     - name: prepare rootfs
       uses: libbpf/ci/prepare-rootfs@master
       with:
-        kernel: ${{ inputs.kernel }}
         project-name: 'libbpf'
         arch: ${{ inputs.arch }}
+        kernel: ${{ inputs.kernel }}
+        kernel-root: '.kernel'
         image-output: '/tmp/root.img'
     # 5. run selftest in QEMU
     - name: Run selftests