executor/gitlab_runner_config: run gateway jobs in the "host" network

This is primarily a workaround for an ipv6-disable-related podman bug that breaks networking in containers (likely because an ipv6 address is set up in the container despite the kernel not supporting ipv6).

That being said, the gateway runner should have access to anything, as it is the only way to automate every sort of maintainance via gitlab (save having direct access to the host).