Since a lot of steps can be uses Tool Environment or Environment variable need to set it before any another steps is process during build.

E.g. I configure M2 Extra Step -> Shell Execute. When I trying to use Tool Environment I have empty value of any variable which I'm trying to use.

In the log I see that tool environment variable was set after executing of a script. For my oppinion we need to set both (tool and another) environments variables before any steps were proceed.