Fix .testing make for regression tests

This commit fixes a bug that target code cannot be built for regression
tests when BUILD is specified with command line arguments. The fix
explicitly specifies BUILD argument for target codebase, avoiding the
variable being passed from the parent make.

.testing/Makefile

@@ -282,7 +282,7 @@ $(BUILD)/%/MOM6: $(BUILD)/%/Makefile FORCE
 
 # Target codebase should use its own build system
 $(BUILD)/target/MOM6: $(BUILD)/target FORCE | $(TARGET_CODEBASE)
-	$(MAKE) -C $(TARGET_CODEBASE)/.testing build/symmetric/MOM6
+	$(MAKE) -C $(TARGET_CODEBASE)/.testing BUILD=build build/symmetric/MOM6
 
 $(BUILD)/target: | $(TARGET_CODEBASE)
 	ln -s $(abspath $(TARGET_CODEBASE))/.testing/build/symmetric $@