Set CIBUILDWHEEL environment variable to 1 during build

test/01_basic/setup.py

@@ -1,5 +1,10 @@
+import os
+
 from setuptools import setup, Extension
 
+if os.environ.get('CIBUILDWHEEL', '0') != '1':
+    raise Exception('CIBUILDWHEEL environment variable is not set to 1')
+
 setup(
     name="spam",
     ext_modules=[Extension('spam', sources=['spam.c'])],