build: fail Windows CI on missing GYP sources

Enable the existing GYP missing-source check in the Windows coverage
workflow so stale references fail project generation instead of
only producing warnings.

Assisted-by: Codex
Signed-off-by: inoway46 <inoueyuya416@gmail.com>
PR-URL: #66787
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: inoway46

.github/workflows/coverage-windows.yml

@@ -88,6 +88,8 @@ jobs:
           rustup --version
       - name: Build
         run: ./vcbuild.bat clang-cl v8temporal
+        env:
+          config_flags: -Gmsvs_error_on_missing_sources=1
       # TODO(bcoe): investigate tests that fail with coverage enabled
       # on Windows.
       - name: Test