exfat: github actions: add apt-get update command

    - uses: actions/checkout@v3

    - name: Download the kernel

      run: |

        sudo apt-get update

        sudo apt-get install libelf-dev wget tar gzip python

        wget https://mirrors.edge.kernel.org/pub/linux/kernel/v4.x/linux-4.1.36.tar.gz

        tar xf linux-4.1.36.tar.gz