ci: [java] automatic kokoro label in and /gcbrun comment (#1965)

[gcf-owl-bot[bot]]

Source-Link: https://github.com/googleapis/synthtool/commit/bd2bae89f70bad380da47fab9ec25985dfb87d67 Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:72f0d373307d128b2cb720c5cb4d90b31f0e86529dd138c632710ae0c69efae3

[diegomarquezp]

/gcbrun