Manually add wheezy security repo key
The buster repository is signed with the wheezy key, which is not present in debian-archive-keyring, causing apt-get to fail and interrupt the image build. Works around Librem5/image-builder!46
The buster repository is signed with the wheezy key, which is not present in debian-archive-keyring, causing apt-get to fail and interrupt the image build. Works around Librem5/image-builder!46