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.

Author: herrwang0

.testing/Makefile

@@ -282,9 +282,10 @@ $(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)
+	$(MAKE) -C $(TARGET_CODEBASE)/.testing BUILD=build build/symmetric/
 	ln -s $(abspath $(TARGET_CODEBASE))/.testing/build/symmetric $@
 
 FORCE: