linux.yml: Get installation of Clang 18 back in sync with apt.llvm.org

Author: hartwork

.github/workflows/linux.yml

@@ -41,7 +41,7 @@ jobs:
           - cc: clang-18
             cxx: clang++-18
             clang_major_version: 18
-            clang_repo_suffix:
+            clang_repo_suffix: -18
     steps:
       - name: Add Clang/LLVM repositories
         if: "${{ contains(matrix.cxx, 'clang') }}"