ci: Make it possible to override the number of threads from the environment

When run locally it might make sense to override this value.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>-ne
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>.

ci/run_ci_locally.sh

@@ -8,8 +8,10 @@ cd ${VIRGL_PATH}
 DOCKER_DRIVER=overlay2
 DOCKER_IMAGE=virglrenderer/ci
 
-# Use slightly less than half of available threads
-NUM_THREADS=$(expr $(expr $(nproc) + 2) / 3)
+if [[ -z $NUM_THREADS ]] ; then 
+    # If not forced use slightly less than half of available threads
+    NUM_THREADS=$(expr $(expr $(nproc) + 2) / 3)
+fi    
 
 # When running the erhm, CI, locally,
 # do use HW based backends, which