Use Ubuntu 20.04 for 'Test coverage' jobs and not for 'Test library'

Author: marco-c

.github/workflows/CICD.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-latest, ubuntu-20.04, windows-latest]
+        os: [macos-latest, ubuntu-latest, windows-latest]
         toolchain: [stable, nightly]
     continue-on-error: true
     steps:
@@ -30,7 +30,7 @@ jobs:
         run: cargo test --lib
   test-coverage:
     name: Test coverage (gcc-${{ matrix.gcc }}, clang-${{ matrix.clang }})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         include: