I broke this by accident in 5031de2, likely by misreading the $0. What
the line was intended to do was change to the directory the script is
located in when the script starts, this allows the script to be started
from any location.
Typically this isn't necessary functionality, but for Mac OS it's
important because if you double-click the script (the typical way to run
it) it is started from the user's home directory. Due to this I also
updated the Mac OS distribution test to run the script in this way so
that it's closer to how a user will run it.
Fixes: #218