container: Add a trace log for when we discard "broken pipe" error

I don't think we're hitting this in coreos/rpm-ostree#4567
but it'd be useful to have a trace message in case.

lib/src/container/unencapsulate.rs

@@ -165,6 +165,7 @@ pub(crate) async fn join_fetch<T: std::fmt::Debug>(
         (Err(worker), Err(driver)) => {
             let text = driver.root_cause().to_string();
             if text.ends_with("broken pipe") {
+                tracing::trace!("Ignoring broken pipe failure from driver");
                 Err(worker)
             } else {
                 Err(worker.context(format!("proxy failure: {} and client error", text)))