flake.lock: Update

Flake lock file updates:

• Updated input 'flake-parts':
    'github:hercules-ci/flake-parts/f20dc5d' (2026-03-01)
  → 'github:hercules-ci/flake-parts/5250617' (2026-05-01)
• Updated input 'flake-parts/nixpkgs-lib':
    'github:nix-community/nixpkgs.lib/c185c7a' (2026-03-01)
  → 'github:nix-community/nixpkgs.lib/f590132' (2026-04-26)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/b40629e' (2026-03-18)
  → 'github:NixOS/nixpkgs/15f4ee4' (2026-04-30)
• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/c807e83' (2026-03-21)
  → 'github:oxalica/rust-overlay/4757db4' (2026-05-02)

Author: github-actions[bot]