Use correct cmake variable for prefix.h.in

[perryprog]

Summary

None

Purpose of change

The auto-generated prefix.h file introduced in #57984 seems to not actually reflect the value of CMAKE_INSTALL_PREFIX, which is the value we actually care about for our prefix.

Describe the solution

This adjusts the prefix.h template to use that variable instead.

Describe alternatives you've considered

None, but anyone more familiar with CMake may catch if there's a better solution here.

Testing

Before this change I was unable to run the game as the configuration directory it would look for would be from a prefix value of "", which resolved to relatively random things in different places (and this can be seen by looking at the contents of prefix.h). After this change, prefix.h correctly has a value and everything builds and runs as expected.

NOTE: I am doubtful that CI will properly catch issues with this change, as it didn't previously: please test this yourself and make sure that the game successfully starts without a launcher and when not in the root directory of the source. (Tiles also helps since the font error makes it obvious something's broken.)

Additional context