Add position information to `while evaluating the attribute` errors in the debugger

[9999years]

Before:

0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
0x600001522598

After:

0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
/nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

   131|
   132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
      |                           ^
   133|     in

[9999years]

TODO: Needs a changelog entry.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-02-28-nix-team-meeting-129/40499/1