tmuxp: fix using workspace dirs other than ~/.tmuxp

[infokiller]

For reference see https://github.com/tmux-python/tmuxp/blob/3ba64262839da6a643a19e70bb990ca3e589ad16/src/tmuxp/workspace/finders.py#L99-L128

• [ ] This compdef is not already available in zsh.
• [ ] This compdef is not already available in their original project.
• [ ] I am the original author, or I have authorization to submit this work.
• [ ] This is a finished work.
• [ ] It has a header containing authors, status and origin of the script.
• [ ] It has a license header or I accept that it will be licensed under the terms of the Zsh license.

[syohex]

Thanks