Update build workflow to use ubuntu-20.04

Fixes this error: The ubuntu-18.04 environment is deprecated, consider switching to ubuntu-20.04(ubuntu-latest),
or ubuntu-22.04 instead. For more details see actions/runner-images#6002

Author: cartland

.github/workflows/android.yml

@@ -25,7 +25,7 @@ jobs:
 
   build:
     name: Build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v1