pixi-build-python backend needs a way to specify the source path

[rgommers]

In my dev setup (https://github.com/rgommers/pixi-dev-scipystack/) I consistently put pixi.toml files one level up from the root of the individual packages. With the pixi-build-rattler-build backend it is possible to point at such a location, because in recipe.yaml one can put an arbitrary source path with:

source:
  path: path-to-dir-with-pyproject.toml

With pixi-build-python I didn't find a way to specify a custom path. Inside pixi.toml this is usually done with cwd = , so I had expected something like this to work:

backend = { "pixi-build-python", version = "0.1.*", cwd = "path-to-dir-with-pyproject.toml" }

or alternatively:

backend = { "pixi-build-python", version = "0.1.*" }
path = "path-to-dir-with-pyproject.toml"

I'll note that the pip invocation does use an $SRC_DIR, but there seems to be no way to set SRC_DIR:

+ $PREFIX/bin/python -m pip install --ignore-installed -vv --no-deps --no-build-isolation $SRC_DIR

[baszalmstra]

I actually think this might apply to all backends!

[package.build]
backend = { name = "foobar", version = "1.*" }
source-root = "../" # A path relative to the directory that contains this file.