python3: add pythonForBuild as parameter, fixes python3Minimal

`pythonForBuild` exists for cross-compilation. When one overrides
python, one needs to ensure pythonForBuild matches.

pkgs/development/interpreters/python/cpython/default.nix

@@ -17,6 +17,7 @@
 # For the Python package set
 , packageOverrides ? (self: super: {})
 , buildPackages
+, pythonForBuild ? buildPackages.${"python${sourceVersion.major}${sourceVersion.minor}"}
 , sourceVersion
 , sha256
 , passthruFun
@@ -63,7 +64,7 @@ let
 
   hasDistutilsCxxPatch = !(stdenv.cc.isGNU or false);
 
-  pythonForBuild = buildPackages.${"python${sourceVersion.major}${sourceVersion.minor}"};
+  inherit pythonForBuild;
 
   pythonForBuildInterpreter = if stdenv.hostPlatform == stdenv.buildPlatform then
     "$out/bin/python"

pkgs/development/interpreters/python/default.nix

@@ -129,6 +129,7 @@ in rec {
   # Minimal versions of Python (built without optional dependencies)
   python3Minimal = (python37.override {
     self = python3Minimal;
+    pythonForBuild = pkgs.buildPackages.python3Minimal;
     # strip down that python version as much as possible
     openssl = null;
     readline = null;