Expire artifacts after 1 days by default

There's no need to let these artifacts pile up since the only ones that
will be relevant to non-programmers are the latest from any given ref.

.gitlab-ci.yml

@@ -8,3 +8,7 @@ variables:
 
 stages:
   - build
+
+default:
+  artifacts:
+    expire_in: 1 day