ci: use Visual Studio for win+meson job on GitHub Workflows

ci: use Visual Studio for win+meson job on GitHub Workflows

In 7304bd2bc39 (ci: wire up Visual Studio build with Meson, 2025-01-22)
we have wired up a new CI job that builds and tests Git with Meson on a
Windows machine. The expectation here was that this build uses the
Visual Studio toolchain to do so, and that is true on GitLab CI. But on
GitHub Workflows it is not the case because we've got GCC in our PATH,
and thus Meson favors that compiler toolchain over Visual Studio's.

Fix this by explicitly asking Meson to use the Visual Studio toolchain.
While this is only really required for GitHub Workflows, let's also pass
the flag in GitLab CI so that we don't implicitly assume the toolchain
that Meson is going to pick.

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>