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/f84bb1621574473d05e6ed79ba57a371d8ad8259' (2023-09-16)
  → 'github:oxalica/rust-overlay/9d34219b9788fb5cde6f5b5759211d606f5131ec' (2023-09-17)

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