Github actions global binary cache

[mathisloge]

Since vcpkg is bundled with the standard github action runners, it would be quite time-saving to have only one global binary cache for the standard runners.

E.g. when using multiple repositories and each repository uses vcpkg, I'm just using the bundled vcpkg version to minimize the build time across the repositories. I also think it would speed up the process and hurdle of integrating vcpkg into a CI test pipeline, as the whole binary caching setup for each repo with personal access tokens etc could be removed.