Don't set ctty on the executed shell process
Don't set ctty on the executed shell process

See https://github.com/golang/go/issues/29458 and
https://go-review.googlesource.com/c/go/+/231638.

Go 1.15 added a check for a child spawning bug, where ctty was set to a
wrong file descriptor.
`Ctty` in `SysProcAttr` should be set to a file descriptor _in a child
process_, but we were setting it to a file descriptor _in a parent
process_. This was working by accident, but is now caught by `os/exec`.

Instead, leave it as 0, which will default it to the file descriptor of
stdin. In our case, stdin is already set to the tty.

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