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

---------

Co-authored-by: Zanie <contact@zanie.dev>
Co-authored-by: Stijn de Gooijer <stijndegooijer@gmail.com>

.github/workflows/release.yaml

@@ -48,7 +48,6 @@ jobs:
           args: --out dist
       - name: "Test sdist"
         run: |
-          rustup default $(cat rust-toolchain)
           pip install dist/${{ env.PACKAGE_NAME }}-*.tar.gz --force-reinstall
           ruff --help
           python -m ruff --help