CI: Update v2 branch CI to be on par with v3 (#286)

* ci: Update CI to be on par with v3

* ci: Update gosop branch for GopenPGP-v2

.github/workflows/android.yml

@@ -4,12 +4,12 @@ on:
   push:
     branches: [ main ]
   pull_request:
-    branches: [ main ]
+    branches: [ main, v2 ]
 
 jobs:
   build:
     name: Build library for Android with gomobile
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - name: Set up JDK 1.8