exec driver DNS problems with network namespace and stub resolver

On many Linux hosts, there's a DNS stub resolver owned by systemd-resolvd listening at 127.0.0.53. In https://github.com/hashicorp/nomad/pull/8982 we implemented a fix for the `exec` driver's DNS to make sure that `/etc/resolv.conf` pointed to the right file. But when the allocation is using `bridge` networking, the stub resolver is inaccessible from the allocation's namespace.

To fix this we might want to do some SNAT iptables magic to forward DNS to the stub resolver if present on Linux. This is a little complicated by our networking story being owned in the client's network hook (that calls CNI) rather than in the driver.