[ci][github] Fix CI stability

* Sometimes apt-get install qemu fails with fetch error, run apt-get
update first to mitigate
* unittest.py often fails due to app shell not started in 0.5s, increase
  timeout

.github/workflows/github-ci-clang.yml

@@ -72,6 +72,7 @@ jobs:
       if: ${{ matrix.project == 'qemu-virt-arm64-test' }}
       shell: bash
       run: |
+        env -i DEBIAN_FRONTEND=noninteractive sudo apt-get update
         env -i DEBIAN_FRONTEND=noninteractive sudo apt-get install -y qemu-system-arm
     - name: unittest
       if: ${{ matrix.project == 'qemu-virt-arm64-test' }}