refactor(tvix/eval): clone the Arc<codemap::File> for the compiler

This disconnects ownership of the `File` reference in a compiler from
the calling scope, which is required for when we implement `import`.

`import` will need to carry an `Rc<RefCell<CodeMap>>` (or maybe, in
the future, Arc) to give us the ability to add new detected code
files at runtime.

Note that the choice of `Arc` over `Rc` here is not ours - it's the
codemap crate's.

Change-Id: I3aeca4ffc167acbd1701846a332d93550b56ba7d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6630
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

corp/tvixbolt/src/main.rs

@@ -256,7 +256,7 @@ fn eval(trace: bool, code: &str) -> Output {
     let result = tvix_eval::compile(
         root_expr,
         Some("/nixbolt".into()),
-        &file,
+        file.clone(),
         tvix_eval::global_builtins(),
         &mut compilation_observer,
     )