Switch NSIS and its dependencies to use python3. * src/nsis.mk: Remove dependency "build-python". Let scons use its preferred version of python. * src/build-scons.mk, src/build-setuptools.mk: Build with python3.

#!/usr/bin/env bash

prog=$1

if [ -z $prog ]; then
  echo "Missing program name" >&2
  exit 1
fi

cat << EOF
@echo off
set BINPATH=%~dp0
%BINPATH%bash -c '$prog %*'
EOF