binary_grub-efi: Allow removal of protected packages

Ubuntu have recently started marking shim-signed as Protected/Important in their Debian packaging, which means apt will refuse to remove it unless you pass extra flags.

This causes live-build to fail in the binary_grub-efi stage when it tries to clean up the packages it has temporarily installed to build the bootloader for the binary stage.

I propose temporarily adding the --allow-remove-essential flag to the apt options during this stage to allow such packages to be removed without failure.

I have tested that this patch allows an Ubuntu focal based iso image to build, whereas the master branch currently can't.

approved this merge request

enabled an automatic merge when the pipeline for bd2a0e97 succeeds

merged