specify shell to use during install, to avoid problems with non-standard shells.

specify shell to use during install, to avoid problems with non-standard shells.

based on github pull request #68 from nulltrek, who noted: "If the system shell
is not syntax compliant with sh (such as fish shell), the install script fails
to execute some lines (like stage2.run:671, fish shell doesn't have the &&
operator) because the "su" command opens a new default shell"