ci/windows: add back build-error detection

This was accidentally broken while refactoring the script.

Fixes: bf3c772e5e ("ci: Improve vs2019 mesa_build.ps1 for remove the need of cmd.exe")
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16965>

.gitlab-ci/windows/mesa_build.ps1

@@ -45,8 +45,13 @@ meson --default-library=shared -Dzlib:default_library=static --buildtype=release
 -Dbuild-tests=true -Dwerror=true -Dwarning_level=2 -Dzlib:warning_level=1 -Dlibelf:warning_level=1 `
 $sourcedir
 
-ninja install -j32
-meson test --num-processes 32
+if ($?) {
+  ninja install -j32
+}
+
+if ($?) {
+  meson test --num-processes 32
+}
 
 $buildstatus = $?
 Pop-Location