flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/aaeaf4574767a0f5ed1787e990139aefcf4db103' (2024-03-30)
  → 'github:nix-community/fenix/c739f83545e625227f4d0af7fe2a71e69931fa4c' (2024-04-12)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/f5a9250147f6569d8d89334dc9cca79c0322729f' (2024-03-29)
  → 'github:rust-lang/rust-analyzer/657b33b0cb9bd49085202e91ad5b4676532c9140' (2024-04-11)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/807c549feabce7eddbf259dbdcec9e0600a0660d' (2024-03-29)
  → 'github:NixOS/nixpkgs/a0c9e3aee1000ac2bfb0e5b98c94c946a5d180a9' (2024-04-12)