radv/ci: reduce the parallelism for vkcts-vangogh

radv/ci: reduce the parallelism for vkcts-vangogh

We are about to enable pre-merge testing for radv-zink on vangogh,
which would mean the steam decks would be used for the following jobs:

 * Mesa pre-merge CI:
  * zink: 3 (~12 minutes)
 * Mesa Post-merge CI:
   * vkcts: 4 (~30 minutes)
   * vkd3d: 1 (~5 minutes)
 * DXVK CI: 1 (takes ~4 hours)

This means we could have 9 jobs running at the same time on steam
decks, despite only having 6 available. By reducing the number of decks
allocated for VKCTS runs from 4 to 2, we get closer to the actual
availability, and since vkd3d is so short + DXVK CI runs so
infrequently, we should never have to wait for a deck for too long!

Unfortunately, with the change of parallelism, a known flake started
failing more consistently, so I added it to the flakes list.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21873>