[CI] multi-repository PR should default as PR target branch and not master

[aHenryJard]

Description

On ci build, the default target branch is hard coded as master. It's not working for PR that target for example release/6.4.0. TheCI script need some update to default to PR target branch.

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. OpenCTI client: { e.g. frontend or python }
4. 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

Actual Output

Additional information

Screenshots (optional)