refactor(tvix/eval): generalize EvalIO container

Don't restrict to a Box<dyn EvalIO>.

There's still one or two places where we do restrict, this will be
solved by b/262.

Change-Id: Ic8d927d6ea81fa12d90b1e4352f35ffaafbd1adf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10639
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/glue/src/builtins/mod.rs

@@ -14,8 +14,8 @@ pub use derivation_error::Error as DerivationError;
 ///
 /// As they need to interact with `known_paths`, we also need to pass in
 /// `known_paths`.
-pub fn add_derivation_builtins(
-    eval: &mut tvix_eval::Evaluation,
+pub fn add_derivation_builtins<IO>(
+    eval: &mut tvix_eval::Evaluation<IO>,
     known_paths: Rc<RefCell<KnownPaths>>,
 ) {
     eval.builtins