Take advantage of BUILD_TESTING Cmake variable for deployment builds

[twsearle]

@twsearle No, there is no change. This change only allows workflow to pass the BUILD_TESTING variable to the cmake tasks. The default is ON/true.

To take full advantage of this change, we should raise a separate PR in orca-jedi to add a if(BUILD_TESTING) block in CMakeLists.txt so it knows what/when to include/exclude CTest related logic.

Originally posted by @matthewrmshin in https://github.com/MetOffice/mo-bundle/issues/647#issuecomment-2980931332