build: Fix the build when '/usr/bin/env python' doesn't work
Invoking the Python interpreter as '/usr/bin/env python' may or may not work. eg., there's no 'python' in the GNOME master SDK anymore. To make it more robust, rely on Meson to find the Python installation, and use that to invoke the Python scripts.
Additionally, use 'files', not 'find_program' to refer to the Python files.