ci: minor nix-in-docker improvements

Cheng Shaorequested to merge
wip/improve-ci-nix into master

This patch makes some minor improvements re nix-in-docker logic in the ci configuration:

  • Update nixos/nix to the latest version
  • Apply $CPUS to cores/max-jobs to avoid oversubscribing while allowing a reasonable degree of parallelism
  • Remove redundant --extra-experimental-features nix-command in later nix shell invocations, it's already configured in /etc/nix/nix.conf

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