fix: Detecting zsh when it is not the "login" shell

[renancaraujo]

This library uses env.SHELL to decide which shell is in use, so it can install the correct completion files.

When someone starts a shell within another (like running "bash" on zsh and vice versa), env.SHELL keeps the value of the original one.

Find a way around that, using zsh on windows bash and setting it as login shell is not the most common practice, most people start it on .bashrc.

Context: https://stackoverflow.com/a/3327022