Support generic variable setting from EnvVars in python - pullrequest

## Context

this issue was identified in #42080 . Effectively the problem is that some packages require the presence of `portaudio.h` registered on the machine to be able to test successfully.

The new package `azure-ai-voicelive` requires that `portaudio` and accompanying dependencies must present to _run tests_, but we can successfully build the wheel without having `portaudio` installed. HOWEVER. the DEPENDENCY `portaudio` MUST have `portaudio.h` available to build.

While we can take advantage of #42693 in the **test** steps of of `azure-ai-voicelive`, the issue is that when generating the API stub, we need to build `portaudio` from source. THAT is blowing up because we haven't installed `portaudio.h` in the `Build Extended` job. We need to offer some amount of customization to the box based on package specific variables.

## The Solution.

We need to create a script at the entrypoint for `Build`, `Build Extended`, `Build Docs`, and `Analyze` jobs that can honor variables that are defined to be necessary either in their platform-matrix or `ci.yml`. This way when we call `install-uv`, we'll install the additional portaudio dependency.