lxdprovision: also turn off apt's retry delay for apt < 3.1.3 in LXD containers

!57 (merged) (#72 (closed)) makes pkgproxyctl setup apt write Acquire::Retries::Delay "false"; when the client's apt is older than 3.1.3. That works around LP #2003851: apt 2.8 on Ubuntu 24.04 hangs indefinitely in an offline apt-get update when one suite misses and the others hit.

The apt proxy config that internal/lxdprovision writes for LXD containers (apt-proxy.conf, bind-mounted by setup lxd and setup snapcraft) does not get it. Those containers are often noble, as in snapcraft's build bases, so they still hang.

The file is mounted read-only from the host and its contents cannot depend on the container's apt version. Decide between:

• writing the setting unconditionally there, since apt 3.1.3+ is unaffected by it except for losing its backoff between retries;
• doing the version check in the pre-start hook.

Also consider the daemon's startup banner, which prints the bare proxy line.