ci: Provide KBUILD_OUTPUT to actions asking for it

As of https://github.com/libbpf/ci/pull/67 a bunch of actions honor
KBUILD_OUTPUT. Doing so will make it possible to separate source code
from build artifacts, which in turn may allow us to support incremental
kernel compilation in CI down the line.
Irrespective of these future changes, actions pertaining the kernel
build now ask for an additional input defining where to store or expect
build artifacts. Provide it.

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

.github/actions/vmtest/action.yml

@@ -77,11 +77,14 @@ runs:
     # 4. prepare rootfs
     - name: prepare rootfs
       uses: libbpf/ci/prepare-rootfs@master
+      env:
+        KBUILD_OUTPUT: '.kernel'
       with:
         project-name: 'libbpf'
         arch: ${{ inputs.arch }}
         kernel: ${{ inputs.kernel }}
         kernel-root: '.kernel'
+        kbuild-output: ${{ env.KBUILD_OUTPUT }}
         image-output: '/tmp/root.img'
     # 5. run selftest in QEMU
     - name: Run selftests