.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/unit-test.yml

@@ -87,7 +87,7 @@ jobs:
           submodules: recursive
           token: ${{ secrets.CHECKOUT_TOKEN || github.token }}
 
-      - uses: ./.github/actions/podman-cleanup
+      - uses: kernelkit/actions/podman-cleanup@v1
 
       - name: Run pre-build script
         if: ${{ inputs.pre_build_script != '' }}
@@ -98,7 +98,7 @@ jobs:
           chmod +x ./pre-build.sh
           bash ./pre-build.sh
 
-      - uses: ./.github/actions/cache-restore
+      - uses: kernelkit/actions/cache-restore@v1
         with:
           target: ${{ env.TARGET }}