nixos/networkmanager: use tmpfiles to setup state directories

[flokli]

Tmpfiles cleanup mentioned in #71298.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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 nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @symphorien

[flokli]

Note: I didn't update nm-setup-hostsdirs, which creates directories in /run/NetworkManager/hostsdirs/, each of these allowing a configured user to drop in host files, and reconfigures the NM-provided dnsmasq to watch these directories.

It seems to be a purely nixos-specific feature introduced in #41715 - I couldn't the PR to master.

While I find such a feature by itself pretty scary and would rather not see it in nixpkgs (we have networking.extraHosts and nss for that) - according to systemd's terminilogy, /run/NetworkManager would be considered a RuntimeDirectory, subject to cleanup on each restart of NetworkManager - so it's probably not the right location for "local overrides" anyways. @rickynils, what are your thoughts?

I opened up #71337 to discuss networking.networkmanager.dynamicHosts.

[worldofpeace]

Thanks for doing this @flokli ❇️