Fix workflow failure for older compilers on Linux

Those older compilers are not available on the `ubuntu-latest`
runner images, because that currently is Ubuntu 22.04. However,
they are available on Ubuntu 20.04, so we can use that instead.

Author: striezel

.github/workflows/linux.yml

@@ -87,7 +87,7 @@ jobs:
           - clang++-9
         build_type: [Debug, Release]
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     env:
       CXX: ${{ matrix.compiler }}