We still need to checkout from git these packages, because most
of them use `replace` in their go.mod, which is only compatible
with installing as main module, but not as an external module.

promtail will use go-systemd when cgo is enabled, but we don't
want to ship a full systemd in slapos, so we build with CGO_ENABLED=0

grafana on the other hand needs to be build with CGO_ENABLED, because
it will use sqlite3. grafana also needs to use yarn to build the
frontend, so it remain a dedicated section with ad-hoc installation
method.