Include rust-toolchain in source distribution (#8414)
**Summary** Simplify CI by ensuring that the source distribution is
always built with the rust version that has been explicitly tested. See
discussion in #8389
Closes #8389
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Co-authored-by: Zanie <contact@zanie.dev>
Co-authored-by: Stijn de Gooijer <stijndegooijer@gmail.com>
