Custom requirements installation fix on vanillas

There is a heavy bug that makes installation fail on vanillas when using `custom_requirements_file`.

I supposes this mistake was made by just copying-paste some snippets.

The main idea: on vanilla versions there is no `inst.stat.md5`-file and it is created much latted. So overall process fails on calclulating `when`-condition. 

There are some other tasks that are successfully executed on these cases (like `install post-requirements`). So I suppose that this is the condition supposed to be when installing custom requirements. :)