lxd backend fails with Ubuntu systems >= 20.04

The lxd backend fails when providing an Ubuntu system starting with focal.

spread.yaml:

```yaml
project: testcraft

path: /testcraft
environment:
PROJECT_PATH: /testcraft
PATH: /snap/bin:$PATH

include:
- tests/spread

backends:
lxd:
systems:
- ubuntu-18.04
- ubuntu-18.10
- ubuntu-19.04
- ubuntu-19.10
- ubuntu-20.04
- ubuntu-22.04
- ubuntu-24.04

suites:
tests/spread/testcraft/:
summary: Tests for testcraft core functionality
```

```terminal
$ lxc version
Client version: 6.2
Server version: 6.2
```

Output:

```terminal
$ spread
2025-02-27 17:41:11 Project content is packed for delivery (24.57MB).
2025-02-27 17:41:11 If killed, discard servers with: spread -reuse-pid=1556125 -discard
2025-02-27 17:41:11 Allocating lxd:ubuntu-18.04...
2025-02-27 17:41:11 Allocating lxd:ubuntu-19.04...
2025-02-27 17:41:11 Allocating lxd:ubuntu-20.04...
2025-02-27 17:41:11 Allocating lxd:ubuntu-22.04...
2025-02-27 17:41:11 Allocating lxd:ubuntu-19.10...
2025-02-27 17:41:11 Allocating lxd:ubuntu-24.04...
2025-02-27 17:41:11 Allocating lxd:ubuntu-18.10...
2025-02-27 17:41:21 Waiting for lxd container spread-45-ubuntu-18-10 to have an address...
2025-02-27 17:41:21 Waiting for lxd container spread-40-ubuntu-19-04 to have an address...
2025-02-27 17:41:22 Waiting for lxd container spread-42-ubuntu-18-04 to have an address...
2025-02-27 17:41:22 Waiting for lxd container spread-43-ubuntu-19-10 to have an address...
2025-02-27 17:41:23 Waiting for lxd container spread-41-ubuntu-24-04 to have an address...
2025-02-27 17:41:23 Waiting for lxd container spread-44-ubuntu-20-04 to have an address...
2025-02-27 17:41:23 Waiting for lxd container spread-46-ubuntu-22-04 to have an address...
2025-02-27 17:41:24 Allocated lxd:ubuntu-18.10 (spread-45-ubuntu-18-10).
2025-02-27 17:41:24 Connecting to lxd:ubuntu-18.10 (spread-45-ubuntu-18-10)...
2025-02-27 17:41:25 Allocated lxd:ubuntu-18.04 (spread-42-ubuntu-18-04).
2025-02-27 17:41:25 Connecting to lxd:ubuntu-18.04 (spread-42-ubuntu-18-04)...
2025-02-27 17:41:25 Allocated lxd:ubuntu-19.04 (spread-40-ubuntu-19-04).
2025-02-27 17:41:25 Allocated lxd:ubuntu-19.10 (spread-43-ubuntu-19-10).
2025-02-27 17:41:25 Connecting to lxd:ubuntu-19.04 (spread-40-ubuntu-19-04)...
2025-02-27 17:41:25 Connecting to lxd:ubuntu-19.10 (spread-43-ubuntu-19-10)...
2025-02-27 17:41:25 Connected to lxd:ubuntu-19.04 (spread-40-ubuntu-19-04) at 10.106.138.113.
2025-02-27 17:41:25 Sending project content to lxd:ubuntu-19.04 (spread-40-ubuntu-19-04)...
2025-02-27 17:41:26 Allocated lxd:ubuntu-24.04 (spread-41-ubuntu-24-04).
2025-02-27 17:41:26 Connecting to lxd:ubuntu-24.04 (spread-41-ubuntu-24-04)...
2025-02-27 17:41:26 Allocated lxd:ubuntu-22.04 (spread-46-ubuntu-22-04).
2025-02-27 17:41:26 Connecting to lxd:ubuntu-22.04 (spread-46-ubuntu-22-04)...
2025-02-27 17:41:26 Executing lxd:ubuntu-19.04:tests/spread/testcraft/version (spread-40-ubuntu-19-04) (1/7)...
2025-02-27 17:41:26 Discarding lxd:ubuntu-19.04 (spread-40-ubuntu-19-04)...
2025-02-27 17:41:27 Allocated lxd:ubuntu-20.04 (spread-44-ubuntu-20-04).
2025-02-27 17:41:27 Connecting to lxd:ubuntu-20.04 (spread-44-ubuntu-20-04)...
2025-02-27 17:41:29 Connected to lxd:ubuntu-18.10 (spread-45-ubuntu-18-10) at 10.106.138.59.
2025-02-27 17:41:29 Sending project content to lxd:ubuntu-18.10 (spread-45-ubuntu-18-10)...
2025-02-27 17:41:30 Executing lxd:ubuntu-18.10:tests/spread/testcraft/version (spread-45-ubuntu-18-10) (2/7)...
2025-02-27 17:41:30 Discarding lxd:ubuntu-18.10 (spread-45-ubuntu-18-10)...
2025-02-27 17:41:30 Connected to lxd:ubuntu-18.04 (spread-42-ubuntu-18-04) at 10.106.138.198.
2025-02-27 17:41:30 Sending project content to lxd:ubuntu-18.04 (spread-42-ubuntu-18-04)...
2025-02-27 17:41:30 Connected to lxd:ubuntu-19.10 (spread-43-ubuntu-19-10) at 10.106.138.13.
2025-02-27 17:41:30 Sending project content to lxd:ubuntu-19.10 (spread-43-ubuntu-19-10)...
2025-02-27 17:41:31 Executing lxd:ubuntu-18.04:tests/spread/testcraft/version (spread-42-ubuntu-18-04) (3/7)...
2025-02-27 17:41:31 Discarding lxd:ubuntu-18.04 (spread-42-ubuntu-18-04)...
2025-02-27 17:41:31 Executing lxd:ubuntu-19.10:tests/spread/testcraft/version (spread-43-ubuntu-19-10) (4/7)...
2025-02-27 17:41:31 Discarding lxd:ubuntu-19.10 (spread-43-ubuntu-19-10)...
2025-02-27 17:42:26 Discarding lxd:ubuntu-24.04 (spread-41-ubuntu-24-04), cannot connect: cannot connect to lxd:ubuntu-24.04 (spread-41-ubuntu-24-04): ssh: handshake failed: ssh: unable to authenticate, attempted methods [none], no supported methods remain
2025-02-27 17:42:26 Discarding lxd:ubuntu-22.04 (spread-46-ubuntu-22-04), cannot connect: cannot connect to lxd:ubuntu-22.04 (spread-46-ubuntu-22-04): ssh: handshake failed: ssh: unable to authenticate, attempted methods [none], no supported methods remain
2025-02-27 17:42:27 Discarding lxd:ubuntu-20.04 (spread-44-ubuntu-20-04), cannot connect: cannot connect to lxd:ubuntu-20.04 (spread-44-ubuntu-20-04): ssh: handshake failed: ssh: unable to authenticate, attempted methods [none], no supported methods remain
2025-02-27 17:42:29 Successful tasks: 4
2025-02-27 17:42:29 Aborted tasks: 3
error: unsuccessful run
```

I believe this distinguishes #151 from #154

Start by reproducing the run with the provided spread.yaml and the tests/spread/testcraft/version entry point. Then inspect the LXD backend's connection and SSH setup for Ubuntu 20.04 and newer. Done means the focal, jammy, and noble systems connect successfully and the spread run completes without aborted tasks.

Written by the indexing model from the issue text.