zink/ci: document a nightly failure

This started failing at some point between a35a12dd38 and 2f7b1e8453,
but the nightly CI was out for a long time in between, so it's hard to
tell what commit it to blame without bisecting. I'm going to leave that
up to someone else for now.

To complicate things further, it seems like this one only fails in the
nightly runs, but not in pre-merge. So we list it as a flake instead of
a consisten failure.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38279>

src/gallium/drivers/zink/ci/zink-venus-lavapipe-flakes.txt

@@ -39,3 +39,5 @@ KHR-GL46.layout_location.sampler_2d_shadow
 # uprev Piglit in Mesa
 spec@ext_framebuffer_multisample@accuracy all_samples depth_draw small depthstencil
 
+# fails in nightly CI, but not in pre-merge?
+KHR-GL46.direct_state_access.vertex_arrays_vertex_buffers_errors