feat(tvix/eval): detect illegally shadowed variables

Nix does not allow things like `let a = 1; a = 2; in a`, but doing it
across depths is allowed.

Change-Id: I6a259f8b01a254b433b58c736e245c9c764641b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6301
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/errors.rs

@@ -32,6 +32,9 @@ pub enum ErrorKind {
     // Unknown variable in dynamic scope (with, rec, ...).
     UnknownDynamicVariable(String),
 
+    // User is defining the same variable twice at the same depth.
+    VariableAlreadyDefined(String),
+
     // Attempt to call something that is not callable.
     NotCallable,