nginx: change location configuration file to /etc/nginx/nginx.conf

[Izorkin]

Motivation for this change

Change location configuration file to /etc/nginx/nginx.conf
Simple viewing of the current configuration.

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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[infinisil]

I prefer not to do this. The current state allows for users to run these services ad-hoc, without any NixOS system even, with nix-build '<nixpkgs/nixos>' -A config.systemd.services.nginx.runner, and I think some people do indeed make use of this with nginx in their nix-shells. This change would make it dependent on a NixOS system and break this very neat functionality.

For easily viewing config files, I prefer to have modules export a configFile option, so people can vim $(nix-build '<nixpkgs/nixos>' -A config.services.foo.configFile to view it, which has the added side benefit of users being able to supply their own config file.