Fix persist_sandbox in Makefile (#2171)

Makefile

@@ -239,7 +239,7 @@ setup-config-prompts:
 		 read -p "Enter a password for the sandbox container: " ssh_password; \
 		 echo "ssh_password=\"$$ssh_password\"" >> $(CONFIG_FILE).tmp; \
 	 else \
-		echo "persist_sandbox=\"$$persist_sandbox\"" >> $(CONFIG_FILE).tmp; \
+		echo "persist_sandbox=$$persist_sandbox" >> $(CONFIG_FILE).tmp; \
 	 fi
 
 	@echo "" >> $(CONFIG_FILE).tmp