chore: expose only a set of env variables

[med8bra]

This is a fix to remove the use of DefinePlugin with process.env as it exposes all env variables in your bundle.
If you inspect https://search.nixos.org/bundle.js you will notice all CI env vars are bundled with your app.

      n.Main.init({
        flags: {
          elasticsearchMappingSchemaVersion: parseInt("42"),
          elasticsearchUrl:
            {
              NODE_ENV: "production",
              SHELL:
                "/nix/store/phqa311klldrcbwid1i22dwnpfc9dnma-bash-5.1-p8/bin/bash",
              npm_config_timing: "",
              npm_config_access: "",
	      ... 
	    }.ELASTICSEARCH_URL || "/backend",