fix(npm): allow multiple NPM_FLAGS during new install execution

[JGAntunes]

🎉 Thanks for submitting a pull request! 🎉

Summary

Follow up to - https://github.com/netlify/build-image/pull/854. With it we indroduced a regression for https://github.com/netlify/build-image/issues/496. This should address it.

---

For us to review and ship your PR efficiently, please perform the following steps:

• [ ] Open a bug/issue before writing your code 🧑‍💻. This ensures we can discuss the changes and get feedback from everyone that should be involved. If you`re fixing a typo or something that`s on fire 🔥 (e.g. incident related), you can skip this step.
• [ ] Read the contribution guidelines 📖. This ensures your code follows our style guide and passes our tests.
• [ ] Update or add tests (if any source code was changed or added) 🧪
• [ ] Update the included software doc (if you updated included software) 📄
• [ ] Update or add documentation (if features were changed or added) 📝
• [ ] Make sure the status checks below are successful ✅

A picture of a cute animal (not mandatory, but encouraged)