feat: steps prepend their install directories to the PATH

[code-steadfast]

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[cuinixam]

I do not think this is flexible enough to cover different use cases from the users. With such a change we only solve the current problem we have with the venv paths to be found before the user python paths.

I would suggest we take a more generic approach:

• add include dirs takes either Path or IncludeDir class which has both the path and an integer for ordering.
• if the user provides a path, we automatically create an IncludeDir with ordering 50
• zero means left, meaning found first in PATH

The paths registered with the same order shall keep the order of when they were registered. For example, three paths A,B,C registered with order 13 will be put in the list based on the order number but keep the ordering A, B, C.

CreateVEnv can register its paths with 0.

We can also consider to have the include_dir_order in the step config, such that an user have control for the ordering without needing to change the step 😄

What do you think about this?