feat(tvix/eval): Support builtins.bitXor

Bitwise "exclusive-or" on integers.

Change-Id: I90a0a15afb3a58662d70e82ea14e48b877476e04
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6550
Autosubmit: wpcarro <wpcarro@gmail.com>
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -115,6 +115,13 @@ fn pure_builtins() -> Vec<Builtin> {
                 })
             })
         }),
+        Builtin::new("bitXor", 2, |args, vm| {
+            force!(vm, &args[0], x, {
+                force!(vm, &args[1], y, {
+                    Ok(Value::Integer(x.as_int()? ^ y.as_int()?))
+                })
+            })
+        }),
         Builtin::new("catAttrs", 2, |mut args, _| {
             let list = args.pop().unwrap().to_list()?;
             let key = args.pop().unwrap().to_str()?;