.gitlab-ci/windows.yml: Don't install libusb or spice packages on 32-bit

.gitlab-ci/windows.yml: Don't install libusb or spice packages on 32-bit

When msys2 updated their libusb packages to libusb 1.0.27, they
dropped support for building them for mingw32, leaving only mingw64
packages.  This broke our CI job, as the 'pacman' package install now
fails with:

error: target not found: mingw-w64-i686-libusb
error: target not found: mingw-w64-i686-usbredir

(both these binary packages are from the libusb source package).

Similarly, spice is now 64-bit only:
error: target not found: mingw-w64-i686-spice

Fix this by dropping these packages from the list we install for our
msys2-32bit build.  We do this with a simple mechanism for the
msys2-64bit and msys2-32bit jobs to specify a list of extra packages
to install on top of the common ones we install for both jobs.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2160
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-id: 20240215155009.2422335-1-peter.maydell@linaro.org