Fix the installer script for non-standard shells.

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.
Calling the su command with the "-s" flag to specify the shell fixes the problem. I called the new variable "THE_SHELL" to avoid issues with environment variables.