Motivation for this change

Before this change, python3.override { ... }.buildEnv would take the
un-overriden python interpreter as basis for the buildEnv. With this
commit, we compute self inside the argument passed to callPackage so the
buildEnv will always reference the python interpreter it belongs to.

As an example where this matters, python3Full is defined as
python3.override { x11Support = true; }. python3Full.buildEnv didn't get that override:

nix-shell -p 'with (import ./. {}); python3Full.buildEnv' --command 'python3 -c "import tkinter"'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File
"/nix/store/s7vw8y02cqzx8bnjxl4bkhlnwvz6ws1s-python3-3.7.6/lib/python3.7/tkinter/__init__.py",
line 36, in <module>
    import _tkinter # If this fails your Python may not be configured
for Tk
ModuleNotFoundError: No module named '_tkinter'

With this commit, that example now works.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.