Windows build changed to one done by the datalad-extensions project using Github actions

This is a cleaner build than on Jenkins because the whole environment setup
is handled by the CI config, at least up to the point of "get a random bag
of Windows bytes".

Also, the Jenkins autobuilder has been intermittently failing for a long
time, not due to any problem with git-annex but just a failure to clean up
directories.

Also, this build runs the test suite, and it is (mostly) passing. Test
suite always failed in the jenkins environment.

Also, this build includes libmagic.

Here is the build workflow used by github actions:
https://github.com/datalad/datalad-extensions/blob/master/.github/workflows/build-git-annex-windows.yaml
The libmagic build has its own workflow:
https://github.com/datalad/file-windows/blob/master/.github/workflows/build.yml

(Also cleaned up some windows build cruft I don't use anymore.)

There is no build-version file to link to. I've opened a todo requesting
one: https://github.com/datalad/datalad-extensions/issues/55