clan: facts generate allow regeneration of facts

Add `--regenerate` flag to `clan facts generate` which allows forcing
the generation of facts, regardless of their current existence.

Examples:
```
clan facts generate [MACHINE] --regenerate
```
or
```
clan facts generate [MACHINE] --service [SERVICE] --regenerate
```

pkgs/clan-cli/clan_cli/machines/update.py

@@ -98,7 +98,7 @@ def deploy_nixos(machines: MachineGroup) -> None:
         env = os.environ.copy()
         env["NIX_SSHOPTS"] = ssh_arg
 
-        generate_facts([machine], None)
+        generate_facts([machine], None, False)
         upload_secrets(machine)
 
         path = upload_sources(".", target)