tf-a-builder: Add "artifact-num-to-keep: 40000"

As of now, this job has 50000+ builds archived, totalling 110+GB.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I639394d7f4db7a1ecdd7323a06ea44f27abe311a
diff --git a/tf-a-builder.yaml b/tf-a-builder.yaml
index 2867b61..08de3e8 100644
--- a/tf-a-builder.yaml
+++ b/tf-a-builder.yaml
@@ -10,6 +10,7 @@
     properties:
         - build-discarder:
             days-to-keep: 15
+            artifact-num-to-keep: 40000
         - authorization:
             !include: authorization.yaml.inc
     parameters: