feat(tvix/eval): Initial resolution of <...> paths

This commit implements (lazy) resolution of `<...>` paths via either the
NIX_PATH environment variable, or the -I command-line flag - both
handled via EvalOptions. As a result, EvalOptions can no longer derive
Copy, meaning we have to clone it at each line of the repl - this is
probably not a huge deal as repl performance is not exactly an inner
loop and we're not cloning very much.

Internally, this works by creating a thunk which pushes a constant
containing the string inside the brackets to the stack, then a new
opcode to resolve that path via the `NixPath`. To get that opcode to
work, we now have to pass in the NixPath when constructing the VM.

This (intentionally) leaves out proper implementation of path resolution
via `findFile` (cppnix just calls whatever identifier called findFile is
in scope!!!) as that's widely considered a bit of a misfeature, but if
we do decide to implement that down the road it likely wouldn't be more
than a few extra ops within the thunk introduced here.

Change-Id: Ibc979b7e425b65cbe88599940520239a4a10cee2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6918
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

corp/tvixbolt/src/main.rs

@@ -286,9 +286,17 @@ fn eval(trace: bool, code: &str) -> Output {
     }
 
     let result = if trace {
-        tvix_eval::run_lambda(&mut TracingObserver::new(&mut out.trace), result.lambda)
+        tvix_eval::run_lambda(
+            Default::default(),
+            &mut TracingObserver::new(&mut out.trace),
+            result.lambda,
+        )
     } else {
-        tvix_eval::run_lambda(&mut NoOpObserver::default(), result.lambda)
+        tvix_eval::run_lambda(
+            Default::default(),
+            &mut NoOpObserver::default(),
+            result.lambda,
+        )
     };
 
     match result {