add debug logging to ascertain the base path the asset server is using (#13820)

# Objective
Explicitly and exactly know what of the environment variables (if any)
are being used/not-used/found-not-found by the
`bevy_asset::io::file::get_base_path()`.

- Describe the objective or issue this PR addresses:
In a sufficiently complex project, with enough crates and such it _can_
be hard to know what the Asset Server is using as, what in the bevy
parlance is its 'base path', this change seems to be the lowest effort
to discovering that.

## Solution

- Added `debug!` logging to the `FileAssetReader::new()` call.

## Testing
See output by making a project and trying something like
`RUST_LOG=bevy_asset::io::file=debug cargo run`
- Ran Bevy's tests.

- How can other people (reviewers) test your changes?: Intentionally
mess with your `env` variables (BEVY_ASSET_ROOT and CARGO_MANIFEST_DIR,
scatter assets about and attempt to (without this change) locate where
it's going wrong.

- Is there anything specific they need to know?: I encountered this
issue in a rather large workspace with many many crates with multiple
nested asset directories.

- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test? Linux.

---

crates/bevy_asset/src/io/file/mod.rs

@@ -6,7 +6,7 @@ mod file_asset;
 #[cfg(not(feature = "multi_threaded"))]
 mod sync_file_asset;
 
-use bevy_utils::tracing::error;
+use bevy_utils::tracing::{debug, error};
 #[cfg(feature = "file_watcher")]
 pub use file_watcher::*;
 
@@ -41,6 +41,10 @@ impl FileAssetReader {
     /// See `get_base_path` below.
     pub fn new<P: AsRef<Path>>(path: P) -> Self {
         let root_path = Self::get_base_path().join(path.as_ref());
+        debug!(
+            "Asset Server using {} as its base path.",
+            root_path.display()
+        );
         Self { root_path }
     }