Motivation for this change

The gcloud tool will start occasionally printing update instructions whenever google publishes a new version. On a nix system, those instructions error out. I looked into it what other package managers do and found that there's a config option for fixing the second problem (the error is replaced with a simple explanation that you've installed the tool using some external package manager), and that a change to a different config file will fix the first.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.