[bug#35126] [PATCH] gnu: rust: Enable parallel tests starting with 1.26.

[Ivan Petkov]

Hi Danny,

When it’s time to merge this in, would you mind also changing the
`generate-checksums` procedure in (guix build cargo-utils) to omit
the `src-name` parameter (as mentioned in #35155)?

If we’re going to be having the CI bootstrap almost the whole rustc
chain for this change, we might as well combine them together and
build all at once!

Thanks,
—Ivan