* Add a Nix expression search path feature. Paths between angle

brackets, e.g.

    import <nixpkgs/pkgs/lib>

  are resolved by looking them up relative to the elements listed in
  the search path.  This allows us to get rid of hacks like

    import "${builtins.getEnv "NIXPKGS_ALL"}/pkgs/lib"

  The search path can be specified through the ‘-I’ command-line flag
  and through the colon-separated ‘NIX_PATH’ environment variable,
  e.g.,

    $ nix-build -I /etc/nixos ...

  If a file is not found in the search path, an error message is
  lazily thrown.

src/nix-env/nix-env.cc

@@ -1253,6 +1253,8 @@ void run(Strings args)
         else if (parseOptionArg(arg, i, args.end(),
                      globals.state, globals.instSource.autoArgs))
             ;
+        else if (parseSearchPathArg(arg, i, args.end(), globals.state))
+            ;
         else if (arg == "--force-name") // undocumented flag for nix-install-package
             globals.forceName = needArg(i, args, arg);
         else if (arg == "--uninstall" || arg == "-e")