Keyring error when executing rootfs script for raspbian

[mthalman]

When building with image caching disabled, the Linux_amd64 src-ubuntu-20.04-coredeps-graph build leg fails with this error:

Running dotnet/arcade's build-rootfs.sh
W: Cannot check Release signature; keyring file not available /usr/share/keyrings/debian-archive-keyring.gpg
E: Keyring-based check was requested; aborting accordingly

This occurs when executing the script: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/main/src/ubuntu/20.04/cross/armv6/10/hooks/pre-build

Build link (internal only)

[lbussell]

[Triage] Tagging @directhex who created this Dockerfile.