InstallDir doesn't work

Make sure InstallDirRegKey isn't setting $INSTDIR. InstallDir only sets the default value of $INSTDIR, in case no other values can be found. The priority of $INSTDIR values, from highest to lowest, is as follows:

1. Value passed on command line with /D
2. Value read from registry key specified by InstallDirRegKey
3. Value specified by InstallDir