Turn NIX_PATH into a config setting

This allows it to be set in nix.conf.

src/libexpr/eval.hh

@@ -339,9 +339,16 @@ struct InvalidPathError : EvalError
 
 struct EvalSettings : Config
 {
+    EvalSettings();
+
+    static Strings getDefaultNixPath();
+
     Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation",
         "Whether builtin functions that allow executing native code should be enabled."};
 
+    Setting<Strings> nixPath{this, getDefaultNixPath(), "nix-path",
+        "List of directories to be searched for <...> file references."};
+
     Setting<bool> restrictEval{this, false, "restrict-eval",
         "Whether to restrict file system access to paths in $NIX_PATH, "
         "and network access to the URI prefixes listed in 'allowed-uris'."};