build(deps): bump actions/cache from 4 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](https://github.com/actions/cache/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yaml

@@ -883,7 +883,7 @@ jobs:
           echo "sha=$(git -C virtio-villain rev-parse HEAD)" >> "$GITHUB_OUTPUT"
       - name: Cache virtio-villain build
         id: villain-cache
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: virtio-villain/target
           key: virtio-villain-${{ runner.os }}-${{ runner.arch }}-${{ steps.villain-src.outputs.sha }}