chore(tvix/eval): fix all current clippy lints

Change-Id: I28d6af8cb408f8427a75d30b9120aaa809a1ea40
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6784
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -162,7 +162,7 @@ impl Error {
                 format!("list index '{}' is out of bounds", index)
             }
 
-            ErrorKind::TailEmptyList => format!("'tail' called on an empty list"),
+            ErrorKind::TailEmptyList => "'tail' called on an empty list".to_string(),
 
             ErrorKind::TypeError { expected, actual } => format!(
                 "expected value of type '{}', but found a '{}'",