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.

pkgs/development/tools/build-managers/cmake/setup-hook.sh

@@ -22,7 +22,7 @@ cmakeConfigurePhase() {
     if [ -z "$dontUseCmakeBuildDir" ]; then
         mkdir -p build
         cd build
-        cmakeDir=..
+        cmakeDir=${cmakeDir:-..}
     fi
 
     if [ -z "$dontAddPrefix" ]; then