fix: clean npm cache on Windows to prevent corruption

Windows GitHub Actions runners sometimes have corrupted npm caches
that cause ECOMPROMISED errors. Clean the cache before running tests
to prevent these failures.

Co-Authored-By: Michael D'Angelo <michael@promptfoo.dev>

Author: claude

.github/workflows/test.yml

@@ -86,6 +86,11 @@ jobs:
       - name: Install package
         run: uv sync
 
+      - name: Clear npm cache on Windows
+        if: runner.os == 'Windows'
+        run: npm cache clean --force
+        continue-on-error: true
+
       - name: Test CLI can be invoked
         uses: nick-fields/retry@v3
         with: