Fix: [AzurePipelines] vcpkg is now preinstalled on Windows images

So instead of integrating our own, we only copy our precompiled
binaries into the right folder.

Add: [AzurePipeline] introducing a release pipeline

This release pipeline creates all the official release binaries,
and publishes them as artifacts. Currently it can only produce
nightlies and custom builds; stable/testing release binaries are
untested.

This commit also splits up the pipeline in small bits, to both
improve readability, and to share code with the CI pipeline where
possible.