CI: don't save all task logs on every build

We currently archive all of the task logs on every job, but this ends up
being between 2-10MB per job and our pipelines have ~100 jobs.

To save space and time, change the log collection to only happen if the
job fails, and explicitly expire them after a week.

This reverts meta-arm 60abe46, but in two years we've not really needed
successful logs, and they can be easily toggled back on if needed in a
branch.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -76,7 +76,8 @@ stages:
 
   artifacts:
     name: "logs"
-    when: always
+    when: on_failure
+    expire_in: 1 week
     paths:
       - $KAS_BUILD_DIR/tmp*/work*/**/temp/log.do_*.*
       - $KAS_BUILD_DIR/tmp*/work*/**/testimage/*