Fix install-pn-test-server for recent Git security issues

[bemoody]

A recent-ish security update (git 1:2.39.5-0+deb12u1) broke the ability to clone a local directory that is owned by another user. This broke ssh-install-pn-test-server:

Cloning into bare repository 'physionet-build.git'...
fatal: detected dubious ownership in repository at '/tmp/physionet-tmp.git'
To add an exception for this directory, call:

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

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

[bemoody]

(I don't fully understand the original issues and I'm fuzzy about whether this is a Git problem or a Debian problem. Regardless, though, I'm pretty annoyed about what seems to be a major regression in a stable package. Obviously, the ability to clone/fetch repositories that you don't trust is fundamental to what Git is meant to do, and to the best of my knowledge Git can securely fetch untrusted repositories via (dumb)HTTP, yet somehow it's impossible to do the same thing via the local filesystem?)