try clean build on failure

Makefile

@@ -25,7 +25,8 @@ build/bootstrap_captive: build/src
 ci:
 	git fetch
 	git reset --hard ${CI_REF}
-	$(MAKE) build
+	mkdir -p build
+	($(MAKE) build || $(MAKE) clean build) > build/ci.log 2> build/ci.err
 
 clean:
 	rm -rf build