GHA/windows: set lookup-only in build-cache jobs

To save a few seconds by not actually restoring the cache, just checking
if there is cache hit.

Follow-up to fb44e44d92 #20456

Closes #20512

.github/workflows/windows.yml

@@ -50,6 +50,7 @@ jobs:
         with:
           path: C:\my-stunnel
           key: ${{ runner.os }}-stunnel-${{ env.STUNNEL_VERSION }}-amd64
+          lookup-only: true
 
       - name: 'install test prereqs (stunnel)'
         if: ${{ steps.cache-stunnel.outputs.cache-hit != 'true' }}