Port CMake testhelpers away from deprecated globalBuildDirIndexKey()
The entry at Config::globalBuildDirIndexKey() is read only once - in CMake::currentBuildDirIndex() - for backwards compatibility. This function attempts to read an entry at Config::buildDirIndexKey() first. Therefore, defaultConfigure() can safely write at this newer key, assuming the current runtime does not change between the call to defaultConfigure() and subsequent calls to CMake::currentBuildDirIndex() (which is hopefully the case). GIT_SILENT