Fix clippy lint

src/advisories/helpers.rs

@@ -567,18 +567,23 @@ where
     // we try to give a more helpful error message about precisely what we
     // tried.
     let res = res.map_err(|_e| {
+        use std::fmt::Write;
+
         let mut msg = "failed to authenticate when downloading repository".to_owned();
         if !ssh_agent_attempts.is_empty() {
             let names = ssh_agent_attempts
                 .iter()
                 .map(|s| format!("`{}`", s))
                 .collect::<Vec<_>>()
                 .join(", ");
-            msg.push_str(&format!(
+
+            write!(
+                &mut msg,
                 "\nattempted ssh-agent authentication, but \
                  none of the usernames {} succeeded",
                 names
-            ));
+            )
+            .unwrap();
         }
         if let Some(failed_cred_helper) = cred_helper_bad {
             if failed_cred_helper {