feat(tvix/eval/io): allow &mut self in EvalIO

It's okay if these calls mutate some internal state inside an
implementation.

Change-Id: I12bb11bde0310778c3da1275696bf7de058863a3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8571
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/vm/mod.rs

@@ -834,7 +834,7 @@ impl<'o> VM<'o> {
                     Value::UnresolvedPath(path) => {
                         let resolved = self
                             .nix_search_path
-                            .resolve(&*self.io_handle, *path)
+                            .resolve(&mut *self.io_handle, *path)
                             .with_span(&frame, self)?;
                         self.stack.push(resolved.into());
                     }