gitlab-ci: always place the build directory at the top

.gitlab/ci/gitlab_ci.cmake

@@ -5,10 +5,10 @@ endif ()
 
 # Set up the source and build paths.
 set(CTEST_SOURCE_DIRECTORY "$ENV{CI_PROJECT_DIR}")
+set(CTEST_BINARY_DIRECTORY "${CTEST_SOURCE_DIRECTORY}/build")
 if (NOT "$ENV{CTEST_SOURCE_SUBDIRECTORY}" STREQUAL "")
   string(APPEND CTEST_SOURCE_DIRECTORY "/$ENV{CTEST_SOURCE_SUBDIRECTORY}")
 endif ()
-set(CTEST_BINARY_DIRECTORY "${CTEST_SOURCE_DIRECTORY}/build")
 
 if ("$ENV{CMAKE_CONFIGURATION}" STREQUAL "")
   message(FATAL_ERROR