Pick up default shell from /etc/passwd if $SHELL not set and not overridden in VS Code settings

[Chuxel]

Right now the default behavior in the Remote - Containers case (and likely in others) is to drop to sh because the $SHELL environment variable is not set.

We've updated dev container definitions to set this environment variable to bash, but a better way is to get the default shell for the current user via /etc/passwd instead.

This likely affects SSH and WSL in some cases as well, but it is most obvious in the containers case due to how images are typically set up.