Create jack-in subshell the unix way on non-Windows

[PEZ]

In an unnecessary attempt to use as similar jack-in command lines between *nix systems and Windows we use pushd to cd to the project root before starting the repl. This directory is then not popped of the stack properly if the process is killed from externally. Not a problem with jack-in, but when copying the command-line and using it manually, this is not very nice. So, better to return to how we used to do it on *nix, with a sub shell.