flake.lock: Update

Flake lock file updates:

• Updated input 'emacs-overlay':
    'github:nix-community/emacs-overlay/3998784d02091a70316eecd435cc6e3e780ff63c' (2023-07-15)
  → 'github:nix-community/emacs-overlay/3fd7df8e48255363c9460e8284f5ec92a3cc4403' (2023-07-22)
• Updated input 'emacs-overlay/nixpkgs-stable':
    'github:NixOS/nixpkgs/fcc147b1e9358a8386b2c4368bd928e1f63a7df2' (2023-07-13)
  → 'github:NixOS/nixpkgs/fa793b06f56896b7d1909e4b69977c7bf842b2f0' (2023-07-20)
• Updated input 'melpa':
    'github:melpa/melpa/4d787fbfd3b555a637cae7c5b352900ffd1f5eaa' (2023-07-11)
  → 'github:melpa/melpa/eee4fe970e0396e443355128e90e622f7ddd8633' (2023-07-17)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/e74e68449c385db82de3170288a28cd0f608544f' (2023-07-13)
  → 'github:NixOS/nixpkgs/9ca785644d067445a4aa749902b29ccef61f7476' (2023-07-21)