Fix: Remove shell.exists() check from get_default_terminal

[ndavd]

Fixes #2006 by using my proposed solution #2006 (comment)

Upon inspecting https://github.com/zellij-org/zellij/blame/main/zellij-server/src/pty.rs#L451 I noticed that the reason why this is happening is because it would need to take the absolute path of the shell. My $SHELL equals zsh, when checking if a path from that exists it will return false. Now, we could just check it and set it to the fallback /bin/sh instead.. But it's not only my system that does this, a far better solution would be to perhaps use the which command to get the full path of the $SHELL output and return that instead.

[imsnif]

Hey @ndavd - while I appreciate the work you've put into this, as I mentioned in my comment in the original issue, I think the best way would be to remove this conditional. We do essentially this same logic you added a little later down the line when we actually open the pane, and if there's a problem we recover from it gracefully and show an error to the user.

If you remove the if entirely (the one linked in my comment), does it work for you?

[ndavd]

Oh I see, that does work for me, yes. I updated the code.

[imsnif]

Great! I think we can also revert the cargo.lock changes, right?

[ndavd]

Yup, done!