feat(pixi-build-rattler-build): add context variable

[toprinse]

Description

This PR is motivated by the fact that, using pixi-rattler-build-backend requires to specify package version in the recipe.yaml rather than into the pixi.toml. So the idea is to add a context variable that can use the version given in the pixi.toml and propagate it into the recipe using PIXI_PACKAGE_VERSION jinja variable.

Before change

in recipe.yml :

context:
  version: 1.2.0

and in pixi.toml:

[package]
name    = "cpp_math"
version = "1.2.0"

Then, after changes :
in recipe.yml :

context:
  version: ${{ PIXI_PACKAGE_VERSION }}

and in pixi.toml:

[package]
name    = "cpp_math"
version = "1.2.0"

Closes #5782

How Has This Been Tested?

• Tested using the cpp_math example refactored for multi-ouptut processing with nanobind (cpp_math.zip)

• Tested using local built pixi executable and PIXI_BUILD_BACKEND_OVERRIDE to use local built build-backend

PIXI_BUILD_BACKEND_OVERRIDE="pixi-build-rattler-build=local-path-build-backend"  && \
./pixi build --path path_to_cpp_math

• Unit test for the inject_pixi_version_into_context function

AI Disclosure

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  • I have tested any AI-generated content in my PR.
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Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.