Build system: fix FTBFS on jenkins due to Git safe.directory violation (refs #20560)

Since git 2.39.5-0+deb12u1 we get this failure on jenkins:

Cloning into '/home/vagrant/amnesia'...
fatal: detected dubious ownership in repository at '/amnesia.git/.git'
To add an exception for this directory, call:

    git config --global --add safe.directory /amnesia.git/.git
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

Me and boyska do not experience this on our local setups, so probably the isoworkers have different access rights.