add clang-13 to CI

Author: bernhardmgruber

Diff for: .github/workflows/ci.yaml

@@ -42,6 +42,9 @@ jobs:
           - name: build-ubuntu-clang12
             CXX: clang++-12
             INSTALL_EXTRA: clang-12
+          - name: build-ubuntu-clang13
+            CXX: clang++-13
+            INSTALL_EXTRA: clang-13
           - name: build-ubuntu-icpc
             CXX: icpc
             INSTALL_ONEAPI: true