tf-a-builder: Add mark-unstable-if-failed to LAVA post-build action

So failure during LAVA submission didn't go unnoticed and didn'tr result
in successful build (which one is not in such case). The issue seen
in https://ci.trustedfirmware.org/job/tf-a-builder/2022523/console
and discussed in
https://linaro.atlassian.net/browse/TFC-437?focusedCommentId=155610

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I83b7802f9b1d9efcc593bb15d8e876be68bcc514
1 file changed