Pin to Ubuntu 20 runner (#550)

Author: a-maurice

.github/workflows/build_linux.yml

@@ -38,7 +38,7 @@ on:
 jobs:
   build_desktop:
     name: build-linux-unity${{inputs.unity_version}}-CPP${{ inputs.firebase_cpp_sdk_version }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false