CPack/WiX: Restore condition for passing build directory to candle search path

Refactoring in commit 48adc29721 (replace "std::string::find(x) == 0"
with cmHasPrefix(), 2020-03-20, v3.18.0-rc1~488^2~7) accidentally
changed `std::string::rfind(x, 0) != 0` to `!cmHasSuffix()` instead of
`!cmHasPrefix()`.

This is currently not an big issue, but leads to the build directory
being included even when no source file outside the build directory is
used in the project.