refactor: Use variables directly in formatted strings (#7284)

made with `cargo clippy --all --fix` then manually reviewed to ensure
this was the only thing that changed.

src/mimefactory.rs

@@ -419,10 +419,7 @@ impl MimeFactory {
                 None
             } else {
                 if keys.is_empty() && !recipients.is_empty() {
-                    bail!(
-                        "No recipient keys are available, cannot encrypt to {:?}.",
-                        recipients
-                    );
+                    bail!("No recipient keys are available, cannot encrypt to {recipients:?}.");
                 }
 
                 // Remove recipients for which the key is missing.