Ubuntu kernel snap build process could use unauthenticated sources

The pc-kernel snap build process hardcoded the --allow-insecure-repositories and --allow-unauthenticated apt options when creating the build chroot environment. This could allow an attacker who is able to perform a MITM attack between the build environment and the Ubuntu archive to install a malicious package within the build chroot. This issue affects pc-kernel versions prior to and including 2019-07-16