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

@@ -12,7 +12,7 @@ on:
 
 jobs:
   GCC:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         build_type: [Debug, RelWithDebInfo, MinSizeRel, Release] 
@@ -41,7 +41,7 @@ jobs:
 
 
   Clang:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         build_type: [Debug, RelWithDebInfo, MinSizeRel, Release] 
@@ -50,11 +50,11 @@ jobs:
       - uses: actions/checkout@v2
       - name: Set up Clang on Ubuntu
         run: |
-          sudo apt-get install -y clang-10
+          sudo apt-get install -y clang-11
 
       - name: CMake
         run: |
-          cmake -S . -B build -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} -DCMAKE_C_COMPILER=clang-10 -DCMAKE_CXX_COMPILER=clang++-10 -DTAU_BUILDINTERNALTESTS=On -DTAU_BUILDTHIRDPARTYTESTS=On
+          cmake -S . -B build -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} -DCMAKE_C_COMPILER=clang-11 -DCMAKE_CXX_COMPILER=clang++-11 -DTAU_BUILDINTERNALTESTS=On -DTAU_BUILDTHIRDPARTYTESTS=On
 
       - name: Build
         run: |