Fix missing kernel compilation and wrong toolchain architecture

[Copilot]

The GitHub Actions workflow configured the build environment but never compiled the kernel, and downloaded ARM64 Clang for x86_64 runners.

Build kernel step missing compilation

Added actual make commands after toolchain setup:

# Build the kernel
if [[ "${{ env.ENABLE_CCACHE }}" == "true" ]]; then
  export USE_CCACHE=1
  export CCACHE_EXEC=$(which ccache)
fi

make ARCH=${{ env.KERNEL_ARCH }} O=out "${DEVICE}_defconfig"
make ARCH=${{ env.KERNEL_ARCH }} O=out ${{ env.EXTRA_CMDS }} -j$(nproc)

Clang architecture mismatch

Changed download URL from aarch64-linux-gnu.tar.xz to x86_64-linux-gnu-ubuntu-22.04.tar.xz to match GitHub Actions runner architecture.

Code style

Fixed indentation on line 48 in matrix generation.

Original prompt

can you fix problems on it

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