ln.connect should not write any .env files

[falexwolf]

While lamin load and ln.setup.load() should keep writing ~/.lamin/current_instance.env, I think that ln.connect() should not persist anything to disk (should not write .env files).

[falexwolf]

Can you also add the ability to pass None, @Koncopd

Meaning we enable the same behavior as in R:

import lamindb as ln

ln.connect()  # <-- this populates ln.setup.settings.instance for just this user session from `current_instance.env`; it errors if this file doesn't exit

This is relevant if people prefer to work with auto-connect switched off.

lamin set --auto-connect false