Fix clan template creation when already in a flake.
Currently we already fail with very clear and descriptive error when
trying to evaluate the template of the flake we are in:
```
Failed to select template 'flake-parts' from flake '/tmp/superclan' (via attribute path: /tmp/superclan#clanInternals.templates.clan."flake-parts")
```
This is undesired behavior.
When we are trying to create a clan with `clan flakes create`.
We can't rely on the fact that the flake we are currently in exports flake templates.
Now we *try* to evaluate the flake we are in upon creation.
If there are no clan templates available, we now will fall back to
builtin templates.
Closes: #4472
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.
To execute the CLI in foreign directories
.#new-machine needs to get tranformed into /path/to/clan#new-machine
Otherwise it might pick-up some random flake that is in scope where the cli started executing
