Fixed minor issue causing setup_linux.sh to crash on Mac


setup_linux.sh crashed on line 10 because of the $we_are_rpi was a null string.
So I changed $we_are_rpi to "False".

This very minor issue should be fixed eventually when the TODO on line 8 is carried out,
but I decided to fix it anyway so that no one else runs into this error when installing ka-lite.