Update to shim 15.4
    
    - Support for revocations via the ".sbat" section and SBAT EFI variable
    - A new unit test framework and a bunch of unit tests
    - No external gnu-efi dependency
    - Better CI
    - No more "shim*-fedora.efi", as a second safety to avoid system vendors
      setting up the scenario for CVE-2020-15705
    
    Resolves: CVE-2020-14372
    Resolves: CVE-2020-25632
    Resolves: CVE-2020-25647
    Resolves: CVE-2020-27749
    Resolves: CVE-2020-27779
    Resolves: CVE-2021-20225
    Resolves: CVE-2021-20233
    
    Signed-off-by: Peter Jones <pjones@redhat.com>