build: Make xorrisofs reproducible with -volume_date

We need this to be after a '--' as '-volume_date' is a xorriso
flag, not a xorrisofs flag. See the respective man pages.

For more details: https://issues.guix.info/issue/35283#2

Makefile.am

@@ -141,7 +141,7 @@ $(APP_DIST_DIR)/Applications:
 $(APP_DIST_EXTRAS): $(APP_DIST_DIR)/$(OSX_APP)/Contents/MacOS/Bitcoin-Qt
 
 $(OSX_TEMP_ISO): $(APP_DIST_EXTRAS)
-	$(XORRISOFS) -D -l -V "$(OSX_VOLNAME)" -no-pad -r -dir-mode 0755 -o $@ dist
+	$(XORRISOFS) -D -l -V "$(OSX_VOLNAME)" -no-pad -r -dir-mode 0755 -o $@ dist -- $(if $(SOURCE_DATE_EPOCH),-volume_date all_file_dates =$(SOURCE_DATE_EPOCH))
 
 $(OSX_DMG): $(OSX_TEMP_ISO)
 	$(DMG) dmg "$<" "$@"