flake.lock: update to support Rust 1.86 (#4027)

## Motivation

We're using Rust 1.86 now, but our `flake.lock` got left behind.

## Proposal

Update it.

## Test Plan

Tested locally.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: Twey