update flake lock, add flake diff, add evaluation json

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

flupdt/flake_eval.py

@@ -32,3 +32,5 @@ def evaluate_output(path: str, output: str) -> str | None:
         raise RuntimeError(out_msg)
     drv = drv_match.group(1)
     logging.debug(f"derivation evaluated to {drv}")
+
+    return drv