Cache the compiler with ccache to speed kernel builds

- Install ccache and wrap the native + arm64 cross compilers via
  /usr/lib/ccache symlinks on PATH; persist CCACHE_DIR / PATH across steps
  through $GITHUB_ENV / $GITHUB_PATH.
- Restore and save ~/.ccache with actions/cache@v4, keyed by arch + kernel
  version with a unique per-run suffix to force a save; restore-keys warm-start
  from a prior build of the same arch (including across point releases).
- Print `ccache -s` after the build. Deliberately do not cache out/ (freshly
  built packages) or the re-cloned kernel source.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -133,6 +133,40 @@ jobs:
         run: |
           sudo apt-get install -y gcc-aarch64-linux-gnu
 
+      - name: 配置 ccache
+        if: env.BUILD_NEEDED == 'true'
+        run: |
+          set -euo pipefail
+          sudo apt-get install -y ccache
+          # Wrap the (cross-)compilers through /usr/lib/ccache symlinks; the build
+          # picks them up because this dir is prepended to PATH below.
+          sudo update-ccache-symlinks || true
+          ccache_bin="$(command -v ccache)"
+          sudo ln -sf "$ccache_bin" /usr/lib/ccache/gcc
+          if [ "$ARCH" = "arm64" ]; then
+            sudo ln -sf "$ccache_bin" /usr/lib/ccache/aarch64-linux-gnu-gcc
+          fi
+          echo "/usr/lib/ccache" >> "$GITHUB_PATH"
+          {
+            echo "CCACHE_DIR=$HOME/.ccache"
+            echo "CCACHE_MAXSIZE=2G"
+            echo "CCACHE_COMPRESS=1"
+          } >> "$GITHUB_ENV"
+
+      - name: 恢复 ccache 缓存
+        if: env.BUILD_NEEDED == 'true'
+        uses: actions/cache@v4
+        with:
+          # Only the compiler cache is worth persisting: the kernel source is
+          # re-cloned each run and out/ holds freshly built packages. A unique key
+          # per run forces a save; restore-keys warm-start from the most recent
+          # prior build of the same arch (then any build of the same arch).
+          path: ~/.ccache
+          key: ccache-${{ matrix.arch }}-${{ needs.preflight.outputs.kernel_version }}-${{ github.run_id }}
+          restore-keys: |
+            ccache-${{ matrix.arch }}-${{ needs.preflight.outputs.kernel_version }}-
+            ccache-${{ matrix.arch }}-
+
       - name: 创建源码目录
         if: env.BUILD_NEEDED == 'true'
         run: mkdir -p ./kernel/linux
@@ -190,6 +224,10 @@ jobs:
             make bindeb-pkg -j$(nproc) LOCALVERSION="$LOCALVERSION_SUFFIX" KDEB_COMPRESS=gzip skipdbg=true
           fi
 
+      - name: ccache 统计
+        if: env.BUILD_NEEDED == 'true'
+        run: ccache -s
+
       - name: 检查 deb 包
         if: env.BUILD_NEEDED == 'true'
         run: |