.github: use ubuntu-18.04 image

.github/workflows/main.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   android-arm:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
     - name: prepare
@@ -27,7 +27,7 @@ jobs:
         export ANDROID_NDK_ABI=23
         make MAKE_JOBS=2 android-arm-release
   linux:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
     - name: prepare