build action: use ubuntu 20.04 for linux builds

Author: alefnull

.github/workflows/build-plugin.yml

@@ -17,7 +17,7 @@ jobs:
         config:        
         - {
             name: Linux,
-            os: ubuntu-22.04,
+            os: ubuntu-20.04,
             prepare-os: sudo apt-get -y --allow-downgrades --allow-remove-essential --allow-change-held-packages update,
             prepare-os2: sudo apt install -y libglu-dev,
             postfix: lin