We have been having an error from Github saying that API rate limit
exceeded for IP. It hits the limit more often if your public IP is
shared by many other collegues.

{"message":"API rate limit exceeded for 216.228.112.22. (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"}

The suggested solution from GitHub is to use gh.exe tool with "auth
login" arguments. It will store a token and allow us to use API more
than the limit set for IP without token.

However, our cmake build doesn't use gh.exe and API is called via REST.
This commit adds an extra header to the HTTP request with the
infomration of the github token if the value is.

Usage: cmake.exe --preset vs2019 -DSLANG_GITHUB_TOKEN=your_token_here