zink: set ZINK_DESCRIPTORS=db for radv jobs

there's no testing for this in ci, and radv has a unique heap/memory
setup which catches allocation regressions

Reviewed-by: Martin Roukala <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20968>

.gitlab-ci/common/generate-env.sh

@@ -115,6 +115,7 @@ for var in \
     VK_DRIVER \
     VK_ICD_FILENAMES \
     VKD3D_PROTON_RESULTS \
+    ZINK_DESCRIPTORS \
     ; do
   if [ -n "${!var+x}" ]; then
     echo "export $var=${!var@Q}"