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.