python dependency: ensure that setuptools doesn't inject itself into distutils

We do not use setuptools for anything, and only lightly use distutils.
Unpredictable issues can occur due to setuptools monkey-patching, which
interferes with our intended use. Tell setuptools to simply never get
involved.

Note: while it's otherwise possible to check if the probe is run using
sys.executable and avoid forking, setuptools unconditionally injects
itself at startup in a way that requires subprocess isolation to
disable.

mesonbuild/dependencies/python.py

@@ -115,7 +115,9 @@ class BasicPythonExternalProgram(ExternalProgram):
 
         with importlib.resources.path('mesonbuild.scripts', 'python_info.py') as f:
             cmd = self.get_command() + [str(f)]
-            p, stdout, stderr = mesonlib.Popen_safe(cmd)
+            env = os.environ.copy()
+            env['SETUPTOOLS_USE_DISTUTILS'] = 'stdlib'
+            p, stdout, stderr = mesonlib.Popen_safe(cmd, env=env)
 
         try:
             info = json.loads(stdout)