ci(build-kernel): set SKIP_MRPROPER instead of SKIP_DEFCONFIG

Use SKIP_MRPROPER to avoid cleaning the build directory entirely, which can speed up incremental builds in the CI workflow.

.github/actions/build-kernel/action.yml

@@ -9,7 +9,7 @@ runs:
       run: |
         set -euo pipefail
         if [ -f "build/build.sh" ]; then
-          SKIP_DEFCONFIG=1 GKI_DEFCONFIG_FRAGMENT="$GITHUB_WORKSPACE/wild_gki.fragment" BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh
+          SKIP_MRPROPER=1 GKI_DEFCONFIG_FRAGMENT="$GITHUB_WORKSPACE/wild_gki.fragment" BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh
         else
           cp "$GITHUB_WORKSPACE/wild_gki.fragment" common/arch/arm64/configs/wild_gki.fragment
           tools/bazel build --config=release --stamp --notrim --defconfig_fragment=//common:arch/arm64/configs/wild_gki.fragment //common:kernel_aarch64_dist