add debug logging to ascertain the base path the asset server is using

[alphastrata]

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.

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[alphastrata]

@alice-i-cecile presumably this one's pretty uncontroversial yeah?