When the qt-build-system was created, based on the cmake-build-system,
some references to cmake have been missed to be changed.
* guix/build-system/qt.scm (qt-build, qt-cross-build)[modules]:
Use qt-build-system, not cmake-build-system. [builder]: Call qt-build,
not cmake-build.
Coauthored-by: Ludovic Courtès <ludo@gnu.org>