gha: Bump to actions/[down|up]load-artifact@v4 (k8s tests on garm)

`Node.js 19` is deprecated. Bump to a new version based on `Node.js 20`.

As explained at [1] :

> The contents of an Artifact are uploaded together into an immutable
> archive. They cannot be altered by subsequent jobs. Both of these
> factors help reduce the possibility of accidentally corrupting
> Artifact files.

This means that artifacts cannot have the same name.

Adapt the `run-k8s-tests-on-garm` workflow accordingly by embedding all
the other `${{ vmm.* }}` fields and `${{ inputs.tag }}` in the artifact
names that would otherwise collide.

Fixes #9245

Signed-off-by: Greg Kurz <groug@kaod.org>

.github/workflows/run-k8s-tests-on-garm.yaml

@@ -89,9 +89,9 @@ jobs:
         run: bash tests/integration/kubernetes/gha-run.sh collect-artifacts
 
       - name: Archive artifacts ${{ matrix.vmm }}
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
-          name: k8s-tests-garm-${{ matrix.vmm }}
+          name: k8s-tests-garm-${{ matrix.vmm }}-${{ matrix.snapshotter }}-${{ matrix.k8s }}-${{ matrix.instance }}-${{ inputs.tag }}
           path: /tmp/artifacts
           retention-days: 1