test: update cases to silence 'var is never mutated' errors

test/cases/compile_errors/nested_vectors.zig

@@ -1,7 +1,7 @@
 export fn entry() void {
     const V1 = @Vector(4, u8);
     const V2 = @Type(.{ .Vector = .{ .len = 4, .child = V1 } });
-    var v: V2 = undefined;
+    const v: V2 = undefined;
     _ = v;
 }