[CI][multi-repository] build on client python that requires an opencti branch not working for push build

[aHenryJard]

Please replace every line in curly brackets { like this } with an appropriate answer, and remove this line.

Description

Working fine for pr build.

But for push build it's not working, ending on master instead of opencti/issue/xxxx.

Discovered on #745

Environment

1. OS (where OpenCTI server runs): { e.g. Mac OS 10, Windows 10, Ubuntu 16.4, etc. }
2. OpenCTI version: { e.g. OpenCTI 1.0.2 }
3. Other environment details:

Reproducible Steps

Steps to create the smallest reproducible scenario:

1. { e.g. Run ... }
2. { e.g. Click ... }
3. { e.g. Error ... }

Expected Output

opencti service start with a dedicated branch

Actual Output

opencti service starts on master branch.

{ Please describe what actually happened. }

Additional information

If not too long, look at taking the PR target branch as default instead of hardcoded "master".