Trigger update beats job on some builds

[jalvz]

Currently there is an update-beats job in CI that runs on master and ensures that apm-server can update to latest beats version.

Because it only runs on master, it is easy to miss / forget. This is a proposal to run it on PRs that are likely to affect apm-server, that is when changes are detected on:

devtools/mage* and libbeat/scripts/Makefile

That should be enough for now. It is important that that stage runs in parallel with others to not impact significantly build times.

Background in #13239 (comment)

cc @jsoriano

[ph]

I would be +1 to include that changes, you might be interested to know that @mikemadden42

[kuisathaverat]

we can make a job that checks for changes in libbeat daily, this job would create a new PR with the new libbeats updated on the APM Server

[kuisathaverat]

This is the job https://beats-ci.elastic.co/job/elastic+apm-server+master+beats-update/

[kuisathaverat]

There is a Job for each branch

[kuisathaverat]

The job downloads the APM Server code and run the script ./script/jenkins/update-beats.sh so it is easy to add this check to the current pipeline or on a downstream job to do not affect the main pipeline.