Fix behaviour of notify CI job when rebase is needed

Description of bug

notify.sh was failing to run (giving a false success) when a MR was made with changes in jO files and in between another MR was merged to master. In that case the CI job had to do a git pull -rebase which was actually failing behind the scenes since git config was not called before the script.

See https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/47486462

This was not caught because the output of git pull -rebase was sent to /dev/null.

Changes introduced

Added git config in the before_script section of the notify job.

Tests

Issues resolved

Closes #236 (closed)