CI fix: Use version 20.04 when building with clang for Ubuntu

This fixes the problem that `clang-10` is not available for newer versions of
Ubuntu. Of course we could upgrade `clang` as well, but let's restore the
functionality as it was, and after that, we can introduce new changes.

By using a fixed version we get a more controlled build environment.

.github/workflows/ubuntu.yml

@@ -41,7 +41,7 @@ jobs:
 
 
   Clang:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         build_type: [Debug, RelWithDebInfo, MinSizeRel, Release]