git-artifacts: use the setup-git-for-windows-sdk Action

This simplifies the workflow dramatically, as well as accelerating it
because the Action recently learned to use the partial clone/parallel
checkout features of Git.

Note that we have to reinstate that `/usr/bin/git` hack (a shell script
that simply redirects to `/mingw64/bin/git.exe`) in the `pkg` job
manually, since we no longer cache the `build-installers` artifact
_after_ installing that hack in `bundle-artifacts`.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho