flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/aaeaf4574767a0f5ed1787e990139aefcf4db103' (2024-03-30)
  → 'github:nix-community/fenix/3247290e1bba55878a2c62d43894d0309d29c918' (2024-04-20)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/f5a9250147f6569d8d89334dc9cca79c0322729f' (2024-03-29)
  → 'github:rust-lang/rust-analyzer/c83d8cf5844fff3d6e243ab408669222059af1c6' (2024-04-19)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/807c549feabce7eddbf259dbdcec9e0600a0660d' (2024-03-29)
  → 'github:NixOS/nixpkgs/fd16bb6d3bcca96039b11aa52038fafeb6e4f4be' (2024-04-20)