fix(tvix/eval): declare locals before marking them initialised

This has no effect yet, other than changing the way in which some
upvalue captures break (that are already not working correctly).

However, after this change the compiler correctly detects
self-recursion and can start emitting the instructions to deal with
this at runtime.

Change-Id: Id3b0ac206c0204739597a4325bcc66f9c806c242
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6309
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -833,9 +833,9 @@ impl Compiler {
                 todo!("nested bindings in let expressions :(")
             }
 
-            self.compile(entry.value().unwrap());
             let name = path.pop().unwrap();
             self.declare_local(entry.attrpath().unwrap().syntax().clone(), &name);
+            self.compile(entry.value().unwrap());
             self.mark_initialised(&name);
         }