sign-efi-sig-list uses PKCS7 for variable updates

When building some software (https://github.com/puzzleos/uefi-dev)
I ran into a problem/bug in efitools 'sign-efi-sig-list'.

The end result in my case was that an attempt to update the PK variable
in uefi (ovmf files from 20.04 with qemu from 20.04) resulted in an
exit code of 26 (EFI_SECURITY_VIOLATION).

    FS0:\> sb_setup.efi
    SB_SETUP: attempting to configure UEFI Secure Boot
    SB_SETUP: system is in Setup Mode
    SB_SETUP: KEK installed
    SB_SETUP: db installed
    SB_SETUP: unable to set the PK variable (26)

sign-efi-sig-list was used to generate an update to PK in the build process.

The fix upstream is https://git.kernel.org/pub/scm/linux/kernel/git/jejb/efitools.git/commit/?id=e57bafc268511ad54598627b663a7ae86bd856f5

Unfortunately it does not easily cherry-pick to 1.8.1 (20.04's version).

There is only a small amount of changes from 1.8.1 to 21.04's version (1.9.2), so the easiest/safest fix may be to just update.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: efitools 1.8.1-0ubuntu2
ProcVersionSignature: Ubuntu 5.8.0-63.71~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-63-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Fri Aug 20 14:55:19 2021
InstallationDate: Installed on 2020-01-15 (582 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
ProcEnviron:
 TERM=screen.xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: efitools
UpgradeStatus: Upgraded to focal on 2020-04-17 (490 days ago)