bug: warnet init fails if user does not create custom network

[josibake]

Steps to reproduce:

mkdir init-test
cd init-test
uv venv
source .venv/bin/activate.fish
warnet init

Which gives me:

❯ warnet init

Creating project structure...
Creating directory: /root/warnet-projects/warnet-snapshots/networks
Copied directory: 6_node_bitcoin
Finished copying files to /root/warnet-projects/warnet-snapshots/networks
Creating directory: /root/warnet-projects/warnet-snapshots/scenarios
Copied file: tx_flood.py
Copied file: ln_init.py
Copied file: signet_miner.py
Copied file: miner_std.py
Copied file: sens_relay.py
Finished copying files to /root/warnet-projects/warnet-snapshots/scenarios
Copied network example files to /root/warnet-projects/warnet-snapshots/networks
Created warnet project structure in /root/warnet-projects/warnet-snapshots
[?] Do you want to create a custom network? (Y/n): n

Traceback (most recent call last):
  File "/root/warnet-projects/warnet-snapshots/.venv/bin/warnet", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/root/warnet-projects/warnet-snapshots/.venv/lib/python3.12/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/warnet-projects/warnet-snapshots/.venv/lib/python3.12/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/root/warnet-projects/warnet-snapshots/.venv/lib/python3.12/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/warnet-projects/warnet-snapshots/.venv/lib/python3.12/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/warnet-projects/warnet-snapshots/.venv/lib/python3.12/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/warnet/src/warnet/project.py", line 325, in init
    new_internal(directory=current_dir, from_init=True)
  File "/root/warnet/src/warnet/project.py", line 309, in new_internal
    f"\nEdit the network files found in {custom_network_path} before deployment if you want to customise the network."
                                         ^^^^^^^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'custom_network_path' where it is not associated with a value