flake.lock: Update

Flake lock file updates:

• Updated input 'circt-src':
    'github:llvm/circt/fa85c6fefbf27e326073a93d455d5224a871b6a7' (2023-08-18)
  → 'github:llvm/circt/8c9734ef90d247d3534673ef0f7ab3f769901c8c' (2023-08-21)
• Updated input 'llvm-submodule-src':
    'github:llvm/llvm-project/5bf8de882ae96a53fdd15eef1682b9168d3d16a2' (2023-08-10)
  → 'github:llvm/llvm-project/cd09089549562c54a57ae1e7bf5bb42e179e2afa' (2023-08-18)
• Updated input 'slang-src':
    'github:MikePopoloski/slang/9bc6f435bcf0e16767fff6d50aa22cd6b1607766' (2023-08-17)
  → 'github:MikePopoloski/slang/5558a00cb679d77dc38a6174c3b4ef02d836e828' (2023-08-20)