ci: don't mask failing targets

If multiple targets are built, any breaking should fail the build, so
stop when any of them fail. Otherwise a further target that passes would
mask the prior failure.

.gitlab/ci/ctest_build.cmake

@@ -51,6 +51,10 @@ foreach (target IN LISTS targets_to_build)
   math(EXPR num_warnings "${num_warnings} + ${num_warnings_target}")
 
   ctest_submit(PARTS Build)
+
+  if (build_result)
+    break ()
+  endif ()
 endforeach ()
 
 if ("$ENV{CMAKE_CONFIGURATION}" MATCHES "doxygen")