[jjbb] filter only for the default branch, PRs and tags (elastic#2389)

* [jjbb] filter only for the default branch, PRs and tags

* [jjbb] skip tags from the regex

Co-authored-by: SylvainJuge <[email protected]>

Co-authored-by: SylvainJuge <[email protected]>

Author: v1v

.ci/jobs/apm-agent-java-mbp.yml

@@ -13,7 +13,7 @@
             discover-pr-forks-trust: permission
             discover-pr-origin: merge-current
             discover-tags: true
-            head-filter-regex: '^(?!stable).*$'
+            head-filter-regex: '(master|PR-.*)'
             notification-context: 'apm-ci'
             repo: apm-agent-java
             repo-owner: elastic