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/lib.rs

@@ -11,7 +11,10 @@ mod tests;
 /// Tell the Evaluator to resolve `<nix>` to the path `/__corepkgs__`,
 /// which has special handling in [tvix_io::TvixIO].
 /// This is used in nixpkgs to import `fetchurl.nix` from `<nix>`.
-pub fn configure_nix_path(eval: &mut tvix_eval::Evaluation, nix_search_path: &Option<String>) {
+pub fn configure_nix_path<IO>(
+    eval: &mut tvix_eval::Evaluation<IO>,
+    nix_search_path: &Option<String>,
+) {
     eval.nix_path = nix_search_path
         .as_ref()
         .map(|p| format!("nix=/__corepkgs__:{}", p))