Port the functionality to build snapshots from Azure Pipelines to GitHub workflows

[dscho]

We successfully ported the process to build release artifacts and to publish them from the Git Artifacts Azure Pipeline (and the corresponding Azure Release Pipeline) to the tag-git/git-artifacts/release-git GitHub workflows.

The Git Artifacts Azure Pipeline also builds Git for Windows' snapshots, though, and there is an Azure Release Pipeline to actually publish them.

This functionality needs to be ported, too, and then we need to make sure that the GitForWindowsHelper GitHub App triggers those builds.

The best approach would probably be to extend the git-artifacts workflow to take on building the snapshot builds, too, maybe by repurposing (or even renaming?) the existing-tag input to take a commit hash for which a light-weight, unsigned tag needs to be created on the fly.

The GitForWindowsHelper GitHub App could then detect when such a snapshot build finishes (identified by the title of the Check Run that is associated with the commit in git-for-windows/git, we would need to make sure that snapshot builds say something like Building snapshot <timestamp>).

As a bonus, we can build on top of this work to build test artifacts via /test-git-artifacts, which would then trigger the same build, but probably only for x86_64 and only the installer, portable and mingit artifacts. We would need to make sure that this gets a different title, to avoid having those snapshots getting uploaded to https://wingit.blob.core.windows.net/files/index.html.