python: fix full builds by referring to the correct interpreter

The package set is an attribute of the interpreter. The function to
build an environment (`buildEnv`/`withPackages`) is part of the
interpreter. The interpreter is passed to itself, and needs to be
updated when overridden.

For cross-compilation we splice the package set, and for that the
various `build/host` interpreters and sets need to be available. We
select these currently through `pkgs.${pythonAttr}`. The `pythonAttr`
attribute was not fixed for `pythonFull`.

https://github.com/NixOS/rfcs/pull/83
https://github.com/NixOS/nixpkgs/pull/104201

We need a better solution for this because this is very brittle.

pkgs/development/interpreters/python/default.nix

@@ -191,6 +191,7 @@ in {
   # Minimal versions of Python (built without optional dependencies)
   python3Minimal = (python38.override {
     self = python3Minimal;
+    pythonAttr = "python3Minimal";
     # strip down that python version as much as possible
     openssl = null;
     readline = null;

pkgs/top-level/all-packages.nix

@@ -11431,38 +11431,46 @@ in
   # These are for compatibility and should not be used inside Nixpkgs.
   pythonFull = python.override {
     self = pythonFull;
+    pythonAttr = "pythonFull";
     x11Support = true;
   };
   python2Full = python2.override {
     self = python2Full;
+    pythonAttr = "python2Full";
     x11Support = true;
   };
   python27Full = python27.override {
     self = python27Full;
+    pythonAttr = "python27Full";
     x11Support = true;
   };
   python3Full = python3.override {
     self = python3Full;
+    pythonAttr = "python3Full";
     bluezSupport = true;
     x11Support = true;
   };
   python36Full = python36.override {
     self = python36Full;
+    pythonAttr = "python36Full";
     bluezSupport = true;
     x11Support = true;
   };
   python37Full = python37.override {
     self = python37Full;
+    pythonAttr = "python37Full";
     bluezSupport = true;
     x11Support = true;
   };
   python38Full = python38.override {
     self = python38Full;
+    pythonAttr = "python38Full";
     bluezSupport = true;
     x11Support = true;
   };
   python39Full = python39.override {
     self = python39Full;
+    pythonAttr = "python39Full";
     bluezSupport = true;
     x11Support = true;
   };