ci: Use FDO_CI_CONCURRENT in run-shader-db.sh as well

Noticed while checking job logs for it being used elsewhere.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6011>

.gitlab-ci/run-shader-db.sh

@@ -13,6 +13,6 @@ cd /usr/local/shader-db
 for driver in freedreno intel v3d; do
     echo "Running drm-shim for $driver"
     env LD_PRELOAD=$LIBDIR/lib${driver}_noop_drm_shim.so \
-        ./run -j 4 ./shaders \
+        ./run -j${FDO_CI_CONCURRENT:-4} ./shaders \
             > $ARTIFACTSDIR/${driver}-shader-db.txt
 done