Reordered 'Stand alone' build steps

After `SeyrenConfigTest.java` was introduced all seyren specific environment variables must be empty to prevent breaking the `mvn clean package` command (i.e. the SeyrenConfigTest tests fail when enviroment vars are set).

Therefore the `mvn clean package` should be run *before* any environment vars are set.
I've updated the README's `Stand alone` section to reflect this necessity.