Log explicit config file path in verbose output (#18353)

When `--config-file` or `UV_CONFIG_FILE` is used, emit a `DEBUG` message
after logging is initialized so users can verify which configuration
file is active.

The existing debug log in `FilesystemOptions::from_file()` fires before
the tracing subscriber is registered, so it is silently dropped.

Closes #17182

---------

Co-authored-by: konstin <konstin@mailbox.org>

crates/uv/src/lib.rs

@@ -362,6 +362,9 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
     )?;
 
     debug!("uv {}", uv_cli::version::uv_self_version());
+    if let Some(config_file) = cli.top_level.config_file.as_ref() {
+        debug!("Using configuration file: {}", config_file.user_display());
+    }
     if globals.preview.all_enabled() {
         debug!("All preview features are enabled");
     } else if globals.preview.any_enabled() {