flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/ab90d06' (2026-06-20)
  → 'github:nix-community/fenix/4febf81' (2026-06-27)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/501826b' (2026-06-20)
  → 'github:rust-lang/rust-analyzer/a775481' (2026-06-26)
• Updated input 'microvm':
    'github:microvm-nix/microvm.nix/225b35c' (2026-06-17)
  → 'github:microvm-nix/microvm.nix/49a3e9f' (2026-06-24)
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/567a49d' (2026-06-16)
  → 'github:nixos/nixpkgs/e73de5b' (2026-06-26)

Author: github-actions[bot]