test(nix): update the label golden now that bindings mint Variables

LABEL_GOLDENS pins the node labels each grammar emits. The nix case is
`{ foo = 1; bar = 2; }` — two module-level scalar bindings — and its golden was
Module:1, recorded when no Nix binding could be named and none was minted.

Both are Variables now, so the golden is Module:1,Variable:2. The mismatch CI
reported is the intended behaviour change, not a regression.

Extraction, pipeline, grammar_labels, grammar_regression and lang_contract:
534 passed, 0 failed.

Signed-off-by: Jason Bowman <jason@json64.dev>

tests/test_grammar_labels.c

@@ -200,7 +200,7 @@ static const LabelGolden LABEL_GOLDENS[] = {
     {"gn", "Module:1"},
     {"just", "Function:1,Module:1"},
     {"hcl", "Class:1,Module:1"},
-    {"nix", "Module:1"},
+    {"nix", "Module:1,Variable:2"},
     {"gomod", "Module:1"},
     {"gotemplate", "Module:1"},
     {"graphql", "Class:1,Field:1,Module:1"},