.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/actions/cache-restore/action.yml

@@ -1,41 +0,0 @@
-name: 'Restore Build Caches'
-description: 'Restore dl/ and .ccache/ caches for Buildroot builds'
-inputs:
-  target:
-    description: 'Build target for cache key differentiation'
-    required: true
-  dl-prefix:
-    description: 'Prefix for dl/ cache key (e.g., "dl", "dl-boot")'
-    required: false
-    default: 'dl'
-  ccache-prefix:
-    description: 'Prefix for .ccache/ cache key (e.g., "ccache", "ccache-boot")'
-    required: false
-    default: 'ccache'
-  enabled:
-    description: 'Set to false to skip cache restoration'
-    required: false
-    default: 'true'
-runs:
-  using: 'composite'
-  steps:
-    - name: Restore Cache of dl/
-      if: ${{ inputs.enabled == 'true' }}
-      uses: actions/cache@v4
-      with:
-        path: dl/
-        key: ${{ inputs.dl-prefix }}-${{ hashFiles('.git/modules/buildroot/HEAD', 'configs/*', 'package/*/*.hash') }}
-        restore-keys: |
-          ${{ inputs.dl-prefix }}-
-          dl-
-
-    - name: Restore Cache of .ccache/
-      if: ${{ inputs.enabled == 'true' }}
-      uses: actions/cache@v4
-      with:
-        path: .ccache/
-        key: ${{ inputs.ccache-prefix }}-${{ inputs.target }}-${{ hashFiles('.git/modules/buildroot/HEAD', 'package/*/*.hash') }}
-        restore-keys: |
-          ${{ inputs.ccache-prefix }}-${{ inputs.target }}-
-          ${{ inputs.ccache-prefix }}-
-          ccache-

.github/actions/podman-cleanup/action.yml

@@ -1,15 +0,0 @@
-name: 'Podman Cleanup'
-description: 'Clean up stale podman state from previous runs'
-runs:
-  using: 'composite'
-  steps:
-    - name: Cleanup podman state
-      shell: bash
-      run: |
-        set -x
-        podman ps -a              || true
-        podman stop -a            || true
-        podman rm -a -f           || true
-        podman volume prune -f    || true
-        podman system prune -a -f || true
-        podman system migrate     || true