`jhack fire -m <other-model` fails

[Gu1nness]

Bug Description

Assuming current model is shard, then running jhack fire --model testing config-server/0 update-status fails.

The command that is crafted is:

juju ssh -m testing config-server/0 /usr/bin/juju-exec -u config-server/0 JUJU_DISPATCH_PATH=hooks/update-status JUJU_MODEL_NAME=shard JUJU_UNIT_NAME=config-server/0 /var/lib/juju/agents/unit-config-server-0/charm/dispatch

The wrong model name is picked.
The reason is twofold:

• The caller does not provide the model name while it has it: here
• The callee does not respect the model provided: here

The two patches should be:

• Pass the model in _build_command to build_event_env
• In build_event_env use the model to create the env dictionary.

Since the model is always known, no need for it to be optional, we could even remove the call to get_current_model in build_event_env.
However, for the sake of consistency and reusability, I would do model = model or get_current_model.

I can provide a PR if you want.

To Reproduce

1. juju add-model modela
2. juju add-model modelb
3. juju deploy -m modela <app-name>
4. jhack fire -m modela <app-name>/0 update-status

Environment

┌──────────────┬──────────────────────────────────────┐
│ jhack        │ 0.4.4.0.29                           │
│ python       │ 3.12.3 (/snap/jhack/632/bin/python3) │
│ juju-* snaps │                                      │
│ microk8s     │ Not Installed.                       │
│ lxd          │ Not Installed.                       │
│ multipass    │ Not Installed.                       │
│ multipassd   │ Not Installed.                       │
│ os           │ Ubuntu Core 24                       │
│ kernel       │ Linux 7.0.0-28-generic x86_64        │
└──────────────┴──────────────────────────────────────┘

Relevant log output

juju ssh -m testing config-server/0 /usr/bin/juju-exec -u config-server/0 JUJU_DISPATCH_PATH=hooks/update-status JUJU_MODEL_NAME=shard JUJU_UNIT_NAME=config-server/0 /var/lib/juju/agents/unit-config-server-0/charm/dispatch
ERROR command terminated with exit code 1

Additional context

No response