I think by rewriting the expression somewhat it becomes possible to use interpreter, thereby preventing the need to go into buildPackages.

This can be tricky though because people might use interpreter in shebangs, in which case the host instead of the build python should be used. There should be a distinction between pythonForBuild and pythonForTarget interpreter. Also in this case it is not applicable because python is build within the same derivation, which would lead to infinite recursion.

Except for the cpython build, people should use always use buildPackages.python37.interpreter at build time and python37.interpreter when they refer to runtime i.e. in shebangs. For pythonPackages I think we should have an alias called pythonForBuild that refers to buildPackages.$python of the current python version.