Add caching for OpenHarmony Native SDK in CI

Introduces a cache step for the OpenHarmony Native SDK in the GitHub Actions workflow to speed up builds and reduce redundant downloads. The SDK is only downloaded if the cache is not hit.

Author: lijy91

.github/workflows/build.yml

@@ -162,7 +162,15 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y ninja-build unzip
 
+      - name: Cache OpenHarmony Native SDK
+        id: cache-ohos-sdk
+        uses: actions/cache@v4
+        with:
+          path: sdk
+          key: ohos-sdk-v5.1.0-linux-x64-${{ runner.os }}
+
       - name: Download OpenHarmony Native SDK
+        if: steps.cache-ohos-sdk.outputs.cache-hit != 'true'
         run: |
           echo $PWD
           wget -q \