Generate build & test matrix from script (#486)

Author: sunmou99

.github/workflows/build_starter.yml

@@ -12,7 +12,7 @@ on:
     inputs:
       unity_version:
         description: 'Unity version'
-        default: '2019'
+        default: '2020'
         required: true
       firebase_cpp_sdk_version:
         description: 'Firebase CPP SDK version to build against (The branch, tag or SHA to checkout). Required if for release'
@@ -103,7 +103,7 @@ jobs:
             echo "::set-output name=release_label::nightly-$(date "+%Y%m%d-%H%M%S")"
             echo "::set-output name=release_version::NoVersion"
             echo "::set-output name=apis::'analytics,auth,crashlytics,database,dynamic_links,firestore,functions,installations,messaging,remote_config,storage'"
-            echo "::set-output name=unity_version::2019"
+            echo "::set-output name=unity_version::2020"
             echo "::set-output name=should_trigger_package::true"
             echo "::set-output name=firebase_cpp_sdk_version::"
             echo "::set-output name=unity_branch::"