Activate the nushell virtualenv overlay correctly (#6766)

The activate.nu file works a little differently than its counterparts in
other shells, and it needs to be invoked as an overlay, rather than
sourced directly into the shell.

This fixes an outstanding issue left over from #6323.


Release Notes:

- (Improved) ...
([#6323](https://github.com/zed-industries/zed/issues/6323)).