.github: use ubuntu-16.04 to run linux actions

.github/workflows/main.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   android-arm:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-16.04
     steps:
     - uses: actions/checkout@v2
     - name: prepare