flake.lock: update rust-overlay

[a-kenji-buildbot]

Automated changes by the update-flake-lock GitHub Action.

Flake lock file updates:

• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/6e8e3332433847cd56186b1f6fc8c47603cf5b46' (2023-10-21)
  → 'github:oxalica/rust-overlay/a3e829c06eadf848f13d109c7648570ce37ebccd' (2023-10-22)

Running GitHub Actions on this PR

GitHub Actions will not run workflows on pull requests which are opened by a GitHub Action.

To run GitHub Actions workflows on this PR, run:

git branch -D update_flake_lock_action
git fetch origin
git checkout update_flake_lock_action
git commit --amend --no-edit
git push origin update_flake_lock_action --force