test_upstream_compat_setup: mark pmaports as safe (MR 8)

Seems to be necessary following some git CVE.

test_upstream_compat_setup.sh

@@ -4,6 +4,7 @@ wget "https://gitlab.com/postmarketOS/ci-common/-/raw/master/install_pmbootstrap
 sh ./install_pmbootstrap.sh pytest
 ln -s /tmp/pmbootstrap/pmb .
 export PMAPORTS="/home/pmos/.local/var/pmbootstrap/cache_git/pmaports"
+git config --global --add safe.directory "$PMAPORTS"
 git -C "$PMAPORTS" checkout "$1"
 pytest -vv test_upstream_compat.py
 touch upstream-compat-"$1".check-passed