Backport of CNI: use tmpfs location for ipam plugin into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24650 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

The below text is copied from the body of the original PR.

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When a Nomad host reboots, the network namespace files in the tmpfs in /var/run are wiped out. So when we restore allocations after a host reboot, we need to be able to restore both the network namespace and the network configuration. But because the netns is newly created and we need to run the CNI plugins again, this create potential conflicts with the IPAM plugin which has written state to persistent disk at /var/lib/cni. These IPs aren't the ones advertised to Consul, so there's no particular reason to keep them around after a host reboot because all virtual interfaces need to be recreated too.

Reconfigure the CNI bridge configuration to use /var/run/cni as its state directory. We already expect this location to be created by CNI because the netns files are hard-coded to be created there too in libcni.

Note this does not fix the problem described for Docker in #24292 because that appears to be related to the netns itself being restored unexpectedly from Docker's state.

Ref: #24292 (comment)
Ref: https://www.cni.dev/plugins/current/ipam/host-local/#files

Testing & Reproduction steps

Run a cluster on a set of VMs, with at least one client. This can't be a server+client because we need to reboot the hosts. You should probably set the server.heartbeat_grace = "5m" to give yourself time to work.

• Run a non-Docker task with network.mode = "bridge". Wait for it to be healthy.
• Reboot the client host.
• Make sure the alloc is restored, the tasks are restarted, and networking works.

Contributor Checklist

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  changelog entry using the make cl command.
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Overview of commits

• 24fa743