reth stage unwind doesn't work using a path to a genesis file

How to reproduce:
Run `reth stage unwind --chain /home/reth/genesis.json num-blocks 100`  using e.g. the genesis file for Bepolia

Expected result:
Command unwinds the node and the node can start syncing 100 blocks back

Actual result:
```
error: Invalid value 'mainnet' for --chain <CHAIN_OR_PATH>: No such file or directory (os error 2).
    [possible values: mainnet,sepolia,holesky,hoodi,dev]
```