Update Ubuntu.yml

Author: mcbarton

.github/workflows/Ubuntu.yml

@@ -65,6 +65,13 @@ jobs:
             cling-version: '1.0'
             llvm_enable_projects: "clang"
             llvm_targets_to_build: "host;NVPTX"
+          - name: ubu24-x86-gcc12-clang-repl-20
+            os: ubuntu-24.04
+            compiler: gcc-12
+            clang-runtime: '20'
+            cling: Off
+            llvm_enable_projects: "clang"
+            llvm_targets_to_build: "host;NVPTX"
           - name: ubu24-x86-gcc12-clang-repl-19
             os: ubuntu-24.04
             compiler: gcc-12