Windows: fix Python path can contain spaces

Fixes #3331 by reverting to a standard Python shebang line for the Python scripts.

Move the shebang line from the full Python path of the build machine to the first Python on the path during runtime. On Windows, the path to Python can contain spaces and this is causing the meson build to fail because it is assuming the first part of the shebang is for the env command and the space separates the executable name.