Vendoring Rust code does not use build_dir

[tiran]

The prepare_new_source function calls vendor_rust with sdist_root_dir as project root directory:

fromager/src/fromager/sources.py

Lines 566 to 586 in 4667c98

	def prepare_new_source(
	ctx: context.WorkContext,
	req: Requirement,
	source_root_dir: pathlib.Path,
	version: Version,
	) -> None:
	"""Default steps for new sources
	- patch sources
	- apply project overrides from settings
	- vendor Rust dependencies
	:func:`~default_prepare_source` runs this function when the sources are new.
	"""
	patch_source(ctx, source_root_dir, req, version)
	pyproject.apply_project_override(
	ctx=ctx,
	req=req,
	sdist_root_dir=source_root_dir,
	)
	vendor_rust.vendor_rust(req, source_root_dir)

This breaks packages that use the build_dir option for an alternative location for pyproject.toml. prepare_new_source should vendor Rust code with pbi.build_dir(sdist_root_dir) instead.

The package primp with build_dir crates/primp-python is affected by the bug, https://github.com/deedy5/primp/tree/v1.1.2/crates/primp-python