cirrus: Enable doc build on msys2/mingw

Currently rST depends on old version sphinx-2.x.
Install it by downloading it.
Remove the need of university mirror, the main repo are recovered.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20201015220626.418-5-luoyonggang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>