Update mise.toml virtualenv location

[bchopson]

Default to VIRTUALENVS_DIR/<virtualenv_name> if VIRTUALENVS_DIR is defined, otherwise .venv

[bchopson]

Something like this should do the trick, although combining mise tera templating with jinja templating could get tricky:
python.virtualenv = '{% if env.VIRTUALENVS_DIR %}{{ env.VIRTUALENVS_DIR }}/{{virtualenv_name}}{% else %}.venv{% endif %}'

Note that virutalenv_name would be templated by copier, while the rest of the template directives should be read at runtime by mise.