build: Use find_program to find codegen.py

meson provides `find_program` to search for local/system scripts and
programs. However, `codegen.py` script, which is provided as a
workaround for `gdbus-codegen` limitations, does not use it.

This patch modifies the way `codegen.py` script is found by using
meson's idiomatic way to find an executable.

https://bugzilla.gnome.org/show_bug.cgi?id=794365