Add environment variables support to Constant path component

[DrBibop]

Simple change that resolves environment variables in Constant path components.

Useful when working on a collaborative project that requires a system user path like AppData. This will allow users to execute pipelines without having to adjust path references in this use case.

[PassivePicasso]

I see the value in this, however it reveals that there are some serious safety and security concerns I have with this which requires more in depth work to resolve.
In this specific case, I think this would also be better solved by introducing a new PathComponent type, EnvironmentalVariable that explicitly calls out its function. It's a cleaner introduction to the environment and improves reporting.
I'm going to pivot on this and improve the setup overall for this and introduce some testing.

[PassivePicasso]

Replaced by #133