test(tvix/eval): add a test for repeated keys in listToAttrs

This came up in the Nix Language channel today and I thought it
warranted a test case.

We did actually implement this correctly.

Change-Id: I4b37c92d06eb6e3a7f59ea3d10af38f2b0a93d53
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7493
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.exp

@@ -0,0 +1 @@
+[ 1 2 ]

tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.nix

@@ -0,0 +1,13 @@
+# Ensure that builtins.listToAttrs returns the first instance of a key.
+
+let
+  inherit (builtins) foldl' listToAttrs;
+
+  input = [ { name = "result"; value = 1; } { name = "result"; value = 2; } ];
+
+  # foldl-based version of listToAttrs with the _opposite_ behaviour.
+  listToAttrs' = list: foldl' ( acc: elem: acc // { ${elem.name} = elem.value; }) {} list;
+in [
+  (listToAttrs input).result
+  (listToAttrs' input).result
+]