Nested `devbox shell` calls should issue a warning

[loreto]

After starting a shell with devbox shell it's easy to forget that you're already inside a devbox, and you might end up calling devbox shell again. Devbox should detect that it's already controlling the shell and issue a warning.

[loreto]

We just released a new version of devbox that should address this issue. If you installed via our install script, you should get the new version automatically next time you run the binary.