Workaround GitHub Action failure

Ubuntu 20.04 and 22.04 workflows are failing due to an error
which is hit when running `apt-get update`.  Until the
problematic package is fixed apply the suggested workaround
described here:

  https://github.com/orgs/community/discussions/47863

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14530

.github/workflows/zloop.yml

@@ -15,8 +15,10 @@ jobs:
         ref: ${{ github.event.pull_request.head.sha }}
     - name: Install dependencies
       run: |
-        sudo apt-get update
-        sudo apt-get -qq upgrade
+        # https://github.com/orgs/community/discussions/47863
+        sudo apt-mark hold grub-efi-amd64-signed
+        sudo apt-get update --fix-missing
+        sudo apt-get upgrade
         sudo xargs --arg-file=${{ github.workspace }}/.github/workflows/build-dependencies.txt apt-get install -qq
         sudo apt-get clean
     - name: Autogen.sh