fix(tvix/eval): address current clippy lints

Note that I've allowed `needless_lifetimes` for the attribute set
iterator, as I find the type easier to understand with these
annotations present.

Change-Id: I33abb17837ee4813076cdb9a87f54bac4a37044e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6373
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -215,6 +215,7 @@ impl NixAttrs {
         self.0.contains(key)
     }
 
+    #[allow(clippy::needless_lifetimes)]
     pub fn iter<'a>(&'a self) -> Iter<KeyValue<'a>> {
         Iter(match &self.0 {
             AttrsRep::Map(map) => KeyValue::Map(map.iter()),