github-actions: Upload built kernel binaries as a downloadable artifact

Add an upload-artifact step that publishes the kernel-<kernelrelease>-<arch>.tar.xz
tarball produced by build_kernel.sh in kernel-container-build, so testers can
download the built kernel (vmlinuz, modules, config, System.map, optional
initramfs) directly from each CI run.

Paired with the matching extraction step in ctrliq/kernel-container-build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 2 people

.github/workflows/kernel-build-and-test-multiarch.yml

@@ -345,6 +345,18 @@ jobs:
         path: output/kernel-build.log
         retention-days: 7
 
+    # Upload kernel binaries tarball (vmlinuz, modules, config) for downstream testing.
+    # Produced by build_kernel.sh in kernel-container-build; the tarball inside is
+    # named kernel-<kernelrelease>-<arch>.tar.xz.
+    - name: Upload kernel binaries tarball
+      uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7.0.1
+      if: always()
+      with:
+        name: kernel-binaries-${{ matrix.arch }}
+        path: output/kernel-*.tar.xz
+        retention-days: 7
+        if-no-files-found: warn
+
     # Upload kselftests qcow2 image
     - name: Upload kselftests qcow2 image
       uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7.0.1