Allow `clan templates list` to function outside a clan.
Currently when bootstrapping a clan and trying to list the templates
it fails as follows:
```
Traceback (most recent call last):
File "/nix/store/pkrsr8zr90bps1fwrl8n74zbb9g038b8-clan-cli/bin/.clan-wrapped", line 9, in <module>
sys.exit(main())
~~~~^^
File "/nix/store/pkrsr8zr90bps1fwrl8n74zbb9g038b8-clan-cli/lib/python3.13/site-packages/clan_cli/cli.py", line 516, in main
args.func(args)
~~~~~~~~~^^^^^^
File "/nix/store/pkrsr8zr90bps1fwrl8n74zbb9g038b8-clan-cli/lib/python3.13/site-packages/clan_cli/templates/list.py", line 11, in list_command
templates = list_templates(args.flake)
File "/nix/store/pkrsr8zr90bps1fwrl8n74zbb9g038b8-clan-cli/lib/python3.13/site-packages/clan_lib/templates/__init__.py", line 20, in list_templates
custom_templates = flake.select("clanInternals.inventoryClass.templatesPerSource")
^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'select'
```
With the change we get the following output:
```
Available 'clan' templates
├── <builtin>
│ ├── default: Initialize a new clan flake
│ ├── flake-parts: Flake-parts
│ └── minimal: for clans managed via (G)UI
Available 'disko' templates
├── <builtin>
│ └── single-disk: A simple ext4 disk with a single partition
Available 'machine' templates
├── <builtin>
│ ├── flash-installer: Initialize a new flash-installer machine
│ └── new-machine: Initialize a new machine
```
Allowing to check for available templates without needing to have a
clan, which improves the bootstrapping experience.
5b5f1975c5