Presently, there are two bugs with `poetry shell` on nushell:

1. The `overlay use` command that poetry sends to the subshell does not
   get run, but merely appears on the command line as if the user had
   typed it themselves. Hitting the enter key is still necessary to
   activate the virtualenv.
2. In the subshell, whenever the user uses a tool that requires
   interactive keyboard input, the input is not echoed on the screen.
   This is because the current code specifically and explicitly turns
   off the terminal echo mode. It is unclear why.

This change fixes both of these problems by changing the invocation of
`nu` to run the `overlay use` command with the `-e` flag, which runs the
given command and then starts an interactive shell.

Disabling echo mode is also removed.