fix(install-script): support $GITHUB_TOKEN

[raffis]

I use the install script in our ci pipeline which happen to be executed quite frequently.
We started to run into gh ratelimiting because of this script.

Run curl -s https://raw.githubusercontent.com/fluxcd/flux2/main/install/flux.sh | sudo bash
[INFO]  Downloading metadata https://api.github.com/repos/fluxcd/flux2/releases/latest
Error: Process completed with exit code 22.

This fix adds support for $GITHUB_TOKEN. If its not set it would just be an anonymous request as it was before.

[relu]

Thank you for your contribution @raffis! 🥇

I've tested this change and it seems to work correctly both when GITHUB_TOKEN is set and not set.

Would you mind also updating the documentation so that other users will be able to more easily find this?

[raffis]

Would you mind also updating the documentation so that other users will be able to more easily find this?

Not at all, I updated the pr 👍🏻