ninja.recipe: Don't assume python 3 is always `python3`

Instead, use `self.config.python_exe` which will always be a usable
Python 3 for running build commands.
7 jobs for ninja-python3 in 66 minutes and 7 seconds (queued for 44 minutes and 5 seconds)
Status Name Job ID Coverage
  Preparation
passed manifest #85019

00:00:21

 
  Build
passed cerbero deps fedora x86_64 #85020

00:37:32

passed cerbero deps windows x86 #85021

00:47:53

passed cerbero deps windows x86_64 #85022

00:25:03

 
  Full Builds
passed build cerbero cross win32 #85024

00:14:37

passed build cerbero cross win64 #85025

00:13:56

passed build cerbero fedora x86_64 #85023

00:14:23