cmake: don't override cmakeDir when set (#40687)

When dontUseCmakeBuildDir is true (aka the default), it overrides
cmakeDir regardless of the package configuration.

While packaging netbee, I needed to both keep dontUseCmakeBuildDir to
true (some hardcoded paths expect the build folder) and set cmakeDir
(since CMakeList.txt was in a subfolder) which proved impossible.
Here is the fix.