* crates.io uses the sparse registry by default, and trying to
explicitly register a sparse registry for it results in errors
* Also set nixConfig to include binary cache
With Cargo 1.68.0, sparse registries were made stable. With sparse
registries, index metadata is stored in an HTTP api rather than in
a git repository.
As relevant to Crane, the relevant changes are mostly that registries
do not always start with `registry+` and mostly start with `registry+`
or `sparse+` depending on whether the registry is sparse or not.
This PR adjusts the core of Crane to differentiate between `registry`
and `sparse`, and adds a new sparse registry factory to make things
easy.
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Co-authored-by: Ivan Petkov <ivanppetkov@gmail.com>
