Support authentication with GitHub in installer script for increased rate-limits

[dan1elhughes]

When running the installer, I get this error:

root@DreamMachine:~/.ssh# sh -c 'DEBUG=1 sh -c "$(curl -sL https://nextdns.io/install)"'
ERROR: Cannot get latest version:
INFO: OS: debian
INFO: GOARCH: arm64
INFO: GOOS: linux
INFO: NEXTDNS_BIN: /usr/bin/nextdns
INFO: INSTALL_RELEASE:
ERROR: Cannot detect running environment.

Which I can pin down to this line in the installer by running the command manually:

curl https://api.github.com/repos/nextdns/nextdns/releases/latest
{"message":"API rate limit exceeded for <ip address> (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}

---

If the installer supported some environment variable $GITHUB_API_TOKEN and passed this as the header to cURL (--header "Authorization: Bearer $GITHUB_API_TOKEN"), I'd be able to install when the GitHub API is rate-limiting me.

[dan1elhughes]

As an aside, I can also bypass this by specifying the latest version by hand instead of having the script read it from GitHub with

NEXTDNS_VERSION="v1.41.0" sh -c 'sh -c "$(curl -sL https://nextdns.io/install)"'