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 Job ID Name Coverage
  Preparation
passed #85019
manifest

00:00:21

 
  Build
passed #85020
cerbero deps fedora x86_64

00:37:32

passed #85021
cerbero deps windows x86

00:47:53

passed #85022
cerbero deps windows x86_64

00:25:03

 
  Full Builds
passed #85024
build cerbero cross win32

00:14:37

passed #85025
build cerbero cross win64

00:13:56

passed #85023
build cerbero fedora x86_64

00:14:23