Unset PYTHON_CFLAGS and PYTHON_CONFIGURE_OPTS for build tests

Makefile

@@ -1,5 +1,9 @@
 .PHONY: test
 
+# Do not pass in user flags to build tests.
+unexport PYTHON_CFLAGS
+unexport PYTHON_CONFIGURE_OPTS
+
 test: bats
 	PATH="./bats/bin:$$PATH" test/run
 	cd plugins/python-build && $(PWD)/bats/bin/bats $${CI:+--tap} test