fix: LocalFs root in test_compatibility is PosixPath('...') (#5261)

I forgot a `str(...)` conversion in #5243. This lead to log lines such
as:

```
Using fs root 'PosixPath('/tmp/test_output/test_backward_compatibility[debug-pg14]/compatibility_snapshot/repo/local_fs_remote_storage/pageserver')' as a remote storage
```

This surprisingly works, creating hierarchy of under current working
directory (`repo_dir` for tests):
- `PosixPath('`
  - `tmp` .. up until .. `local_fs_remote_storage`
    - `pageserver')`

It should not work but right now test_compatibility.py tests finds local
metadata and layers, which end up used. After #5172 when remote storage
is the source of truth it will no longer work.

test_runner/regress/test_compatibility.py

@@ -325,8 +325,8 @@ def prepare_snapshot(
 
     # For each pageserver config, edit it and rewrite
     for config_path, pageserver_config in path_to_config.items():
-        pageserver_config["remote_storage"]["local_path"] = LocalFsStorage.component_path(
-            repo_dir, RemoteStorageUser.PAGESERVER
+        pageserver_config["remote_storage"]["local_path"] = str(
+            LocalFsStorage.component_path(repo_dir, RemoteStorageUser.PAGESERVER)
         )
 
         for param in ("listen_http_addr", "listen_pg_addr", "broker_endpoint"):