Debian, apt-get: how do I ignore NO_PUBKEY for good?

I have a qemu in which I have debian-10.0-powerpc-NETINST-1 plus sid as package source. I need powerpc to as a reference big endian architecture (maybe there's something easier?) My problem is that trying to install packages end me up with NO_PUBKEY errors, which I can't fight: apt install debian-archive-keyring did not do anything about it and I can't install gpg since I can't install **anything** due to broken apt-get with obsolete database which gives 404 for most of files. Please tell me any options to ignore NO_PUBKEY for good. Since it's a throwaway qemu I don't care in the slightest, just let me ignore the stuff and install packages.