CI: Remove Debian backports

The latest Debian 11 image includes bullseye-backports as a default
repository in the /etc/apt/sources.list.  However, this repository
has gone end of life which effectively breaks the default install.

We shouldn't need anything in backports so lets unconditionally
remove backports on all Debian builders to resolve the issue.

Reviewed-by: George Melikov <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #17569

Author: behlendorf

.github/workflows/scripts/qemu-3-deps-vm.sh

@@ -28,6 +28,7 @@ function debian() {
   export DEBIAN_FRONTEND="noninteractive"
 
   echo "##[group]Running apt-get update+upgrade"
+  sudo sed -i '/[[:alpha:]]-backports/d' /etc/apt/sources.list
   sudo apt-get update -y
   sudo apt-get upgrade -y
   echo "##[endgroup]"