patchShebangs: rewrite ".../bin/env python" to /nix/store/.../python

Currently "/usr/bin/env python" is rewritten to "/nix/store/.../env
python". That doesn't really improve anything because the interpreter
still have to be located in $PATH at runtime.

The result is that many nix package expressions do .../bin/env fixup
themselves.

Instead of everyone having to do this patching locally, add the
functionality to the standard environment patchShebangs function so that
everyone can benefit.

WARNING: Rebuilds everything. (Merge to stdenv-updates?)