hack/openbsd: use cdn.openbsd.org for PKG_PATH

[crazy-max]

relates to https://github.com/docker/buildx/actions/runs/22510011639/job/65378564119?pr=3678#step:6:73

==> default: Running provisioner: init (shell)...
default: Running: inline script
default: ++ export PKG_PATH=https://mirrors.ocf.berkeley.edu/pub/OpenBSD/7.5/packages/amd64/
default: ++ PKG_PATH=https://mirrors.ocf.berkeley.edu/pub/OpenBSD/7.5/packages/amd64/
default: ++ pkg_add -x git
default: https://mirrors.ocf.berkeley.edu/pub/OpenBSD/7.5/packages/amd64/: no such dir
default: Can't find git