Fix failing Clang builds on GitHub Actions (#80, thanks @striezel)

A while ago the 'ubuntu-latest' image moved from Ubuntu 22.04 to
Ubuntu 24.04. See <actions/runner-images#10636>.
However, Ubuntu 24.04 does not have packages for Clang 11 and 12
anymore, thus the Clang jobs fail when they try to install the
corresponding compilers. Therefore, the workflow is changed to
use 'ubuntu-22.04' instead of 'ubuntu-latest'.

Furthermore, a job for Clang 19 (latest Clang version available
on Ubuntu 24.04) is added to run tests with a newer version, too.

Author: striezel

.github/workflows/ci.yml

@@ -26,8 +26,14 @@ jobs:
   clang:
     strategy:
       matrix:
-        version: [11, 12]
-    runs-on: ubuntu-latest
+        include:
+          - version: 11
+            os: 'ubuntu-22.04'
+          - version: 12
+            os: 'ubuntu-22.04'
+          - version: 19
+            os: 'ubuntu-24.04'
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - name: Install Clang ${{ matrix.version }}