feat(rattler-build): Add config option for custom recipe path

[jackm97]

Summary

Add a configuration option to pixi-build-rattler-build to specify a custom path to the recipe file, rather than only searching the hardcoded locations.

Use Case

When building multi-output recipes in a pixi workspace, it's common to have:

• One shared recipe.yaml with multiple outputs (e.g., mylib-libs, mylib-devel, mylib)
• Multiple sub-package pixi.toml files, each with [package] name = "mylib-libs" etc.

The current directory structure requires symlinks as a workaround:

packages/mylib/
├── pixi.toml                  # Workspace
├── recipe/
│   └── recipe.yaml            # Single recipe with 3 outputs
├── libs/
│   ├── pixi.toml              # [package] name="mylib-libs"
│   └── recipe -> ../recipe    # Symlink workaround
├── devel/
│   ├── pixi.toml              # [package] name="mylib-devel"  
│   └── recipe -> ../recipe    # Symlink workaround
└── toolkit/
    ├── pixi.toml              # [package] name="mylib"
    └── recipe -> ../recipe    # Symlink workaround

Proposed Solution

Add a recipe-path config option:

# libs/pixi.toml
[package]
name = "mylib-libs"
version = "1.0.0"

[package.build]
backend = { name = "pixi-build-rattler-build", version = "*" }

[package.build.config]
recipe-path = "../recipe/recipe.yaml"  # or "../recipe" for directory

This would eliminate the need for symlinks and make the configuration more explicit.

Current Behavior

The backend only searches these hardcoded paths relative to the manifest:

1. recipe.yaml
2. recipe.yml
3. recipe/recipe.yaml
4. recipe/recipe.yml

(From rattler_build.rs)

Alternatives Considered

• Symlinks: Works but feels like a workaround, not explicit in config
• Duplicate recipes: Defeats the purpose of multi-output recipes
• Flat structure: Loses the logical grouping of sub-packages

Additional Context

This came up while building Intel DPC++/SYCL compiler packages where we have a single complex recipe producing sycl-toolkit-libs, sycl-toolkit-devel, and sycl-toolkit outputs, managed as separate packages in a pixi workspace.

[baszalmstra]

Does https://pixi.sh/latest/build/package_source/ help?