ci: update cache action since v2 is now hard deprecated

Signed-off-by: Liam McLoughlin <lmcloughlin@google.com>

.github/workflows/build.yml

@@ -26,7 +26,7 @@ jobs:
 
       - name: Setup emsdk cache
         id: cache-emsdk
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{env.EM_CACHE_FOLDER}}
           key: emsdk-${{env.EM_VERSION}}-${{ runner.os }}