`--open` does not work with error_index_generator

### Summary

The `--open` flag does nothing in:

```sh
./x doc src/tools/error_index_generator --open
```

### Expected behaviour

Opens a web browser.

### Actual behaviour

Doesn't open a web browser.

### Bootstrap configuration (bootstrap.toml)

Default

### Operating system

Any

### HEAD

401ae55427522984e4a89c37cff6562a4ddcf6b7
