Explicitly specify Qemu image path to use

The path to the file system image used by our invocation of Qemu is
currently hard coded to /tmp/root.img somewhere in a different
repository. With
da44c0b6ee
landed we have the option of specifying it explicitly from here. Let's
do just that, so that we can remove the default value from libbpf/ci
altogether.

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

.github/actions/vmtest/action.yml

@@ -79,6 +79,7 @@ runs:
         kernel: ${{ inputs.kernel }}
         project-name: 'libbpf'
         arch: ${{ inputs.arch }}
+        image-output: '/tmp/root.img'
     # 5. run selftest in QEMU
     - name: Run selftests
       uses: libbpf/ci/run-qemu@master