2configs.nginx: move temp dirs out of /tmp
nixpkgs nginx is compiled with /tmp/nginx_* as temp paths. Combined
with systemd PrivateTmp, host-side cleanup of /tmp can leave nginx
with missing temp dirs after restart, breaking large POSTs (e.g.
matrix /keys/upload) and federation PUTs. Move to /var/cache/nginx,
which is auto-created via the systemd unit's CacheDirectory=nginx.
Assisted-by: Claude Code (claude-opus-4-7)