fix(Makefile): evaluate composer prerequisits during build

[max-nextcloud]

📝 Summary

ifeq (, $(wildcard $(BUILD_TOOLS_DIR)/composer.phar)) is evaluated before any targets are build. Therefore the file still exists. It is only removed later when the dist-clean target is build.

Use a file prerequisit instead. This is evaluated when the target is build.

🖼️ Screenshots

🏚️ Before	🏡 After

🏁 Checklist

• [x] Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• [x] Sign-off message is added to all commits
• [x] Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• [x] Documentation is not required