Merge pull request #8615 from SiliconLabs/dev_9.0

Workaround for Silabs builds intermittently fail in GitHub Actions CI

Author: dhalbert

ports/silabs/Makefile

@@ -139,7 +139,7 @@ $(OUTPUT_DIR)/firmware.bin: $(SILABS_BUILD)/$(PROJECTNAME).Makefile $(OUTPUT_DIR
 endif
 
 $(SILABS_BUILD)/$(PROJECTNAME).Makefile: | $(HEADER_BUILD)/mpversion.h
-	+@$(MAKE) --no-print-directory slc-generate
+	+@$(MAKE) -j1 --no-print-directory slc-generate
 
 $(OUTPUT_DIR)/firmware.out: $(SILABS_BUILD)/pin_functions.h $(SILABS_BUILD)/pins.c $(OBJ) $(OBJS) $(LIB_FILES)
 	$(STEPECHO) 'Linking $(OUTPUT_DIR)/firmware.out'