clarify SALSA_CI_EXTRA_REPOSITORY

Looks like this needs to use a .pgp instead of a .gpg keyring nowadays ...

https://salsa.debian.org/nvidia-team/nvidia-nccl/-/jobs/8263288#L45

But given the recent documentation change for SALSA_CI_COMPONENTS saying that it is used for build jobs, too, the question arises whether SALSA_CI_EXTRA_REPOSITORY is still needed for "normal" builds using contrib or non-free.

In order to make it more clear in the logs which package resources are being used, perhaps you could also add after

apt-get update && eatmydata apt-get upgrade -y

this command:

apt-cache policy

At that point all foreign architectures and extra repositories should be enabled and refreshed, so this should show accurate information about repositories and pins.