[red-knot] knot benchmark: fix `--knot-path` arg (#13923)
## Summary
Previously, this would fail with
```
AttributeError: 'str' object has no attribute 'is_file'
```
if I tried to use the `--knot-path` option. I wish we had a type checker
for Python*.
## Test Plan
```sh
uv run benchmark --knot-path ~/.cargo-target/release/red_knot
```
\* to be fair, this would probably require special handling for
`argparse` in the typechecker.
