Installer: Fix association of .sh files due to a GUID typo

We also need to remove the "createvalueifdoesntexist" flag so that the
fixed GUID is written to the Registry in case of an update installation.

This issue was reported on the mailing list at

https://groups.google.com/d/msg/msysgit/dd1qftVKGME/UdQtb3p1dfwJ

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>