build(nix): update rust toolchain hash

[mrcjkb]

Fixes #2989.

The nix build fails, because the sha256 in the toolchain fetcher doesn't match.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

Important

Before merging this, we should probably investigate why the hash has changed. It could be a sign of a supply chain attack.

[mrcjkb]

Looking at https://static.rust-lang.org/dist/channel-rust-1.91.toml:

manifest-version = "2"
date = "2025-11-10"

so this is just a manifest update and should be safe to merge.