ci: only run win+VS build & tests in Git for Windows' fork

ci: only run win+VS build & tests in Git for Windows' fork

It has been a frequent matter of contention that the win+VS jobs not
only take a long time to run, but are also more easily broken than the
other jobs (because they do not use the same `Makefile`-based builds as
all other jobs), and to make matters worse, these breakages are also
much harder to diagnose and fix than other jobs', especially for
contributors who are happy to stay away from Windows.

The purpose of these win+VS jobs is to maintain the CMake-based build
of Git, with the target audience being Visual Studio users on Windows
who are typically quite unfamiliar with `make` and POSIX shell
scripting, but the benefit of whose expertise we want for the Git
project nevertheless.

The CMake support was introduced for that specific purpose, and already
early on concerns were raised that it would put an undue burden on
contributors to ensure that these jobs pass in CI, when they do not have
access to Windows machines (nor want to have that).

This developer's initial hope was that it would be enough to fix win+VS
failures and provide the changes to be squashed into contributors'
patches, and that it would be worth the benefit of attracting
Windows-based developers' contributions.

Neither of these hopes have panned out.

To lower the frustration, and incidentally benefit from using way less
build minutes, let's just not run the win+VS jobs by default, which
appears to be the consensus of the mail thread leading up to
https://lore.kernel.org/git/xmqqk0311blt.fsf@gitster.g/

Since the Git for Windows project still needs to at least try to attract
more of said Windows-based developers, let's keep the jobs, but disable
them everywhere except in Git for Windows' fork. This will help because
Git for Windows' branch thicket is "continuously rebased" via automation
to the `shears/maint`, `shears/main`, `shears/next` and `shears/seen`
branches at https://github.com/git-for-windows/git. That way, the Git
for Windows project will still be notified early on about potential
breakages, but the Git project won't be burdened with fixing them
anymore, which seems to be the best compromise we can get on this issue.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>