fix: avoid use of deprecated child() fn (#998)

### Description

I get a compilation warning:

```
warning: use of deprecated method `object_store::path::Path::child`: use .join() or .clone().join() instead
   --> crates/cli/src/lib.rs:596:34
    |
596 | ...                   path.child(file_name),
    |                            ^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

warning: `rustac` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4m 15s
```

The implementation of `child()` just does `clone().join()`, and we do
need the `clone()`, or a wider refactoring.

### Checklist

Delete any checklist items that do not apply (e.g. if your change is
minor, it may not require documentation updates).

- [x] Pull request title follows [conventional
commits](https://www.conventionalcommits.org/en/v1.0.0/)
- [x] Pre-commit hooks pass (`prek run --all-files`)

Author: bradh

crates/cli/src/lib.rs

@@ -593,7 +593,7 @@ impl Rustac {
                     );
                     store
                         .put_format(
-                            path.child(file_name),
+                            path.clone().join(file_name),
                             stac::ItemCollection::from(items),
                             format,
                         )