.github: move local actions to kernelkit/actions repo

Fixes a regression from 0c11cea where local composite actions
(.github/actions/) were referenced with relative paths. When spins
reuse our reusable workflows, GitHub resolves relative action paths
in the context of the calling repo, not infix — breaking the build.

Replace all uses of ./.github/actions/{cache-restore,podman-cleanup}
with kernelkit/actions/{cache-restore,podman-cleanup}@v1 (new repo).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/build-boot.yml

@@ -68,7 +68,7 @@ jobs:
 
           echo "Building ${defconfig}_defconfig, version ${version}-${rev}, artifact ${archive} ..."
 
-      - uses: ./.github/actions/cache-restore
+      - uses: kernelkit/actions/cache-restore@v1
         with:
           target: ${{ matrix.defconfig }}
           dl-prefix: dl-boot