Include clang-20 in automated builds

Author: mutability

.github/workflows/build.yaml

@@ -15,13 +15,16 @@ jobs:
           - gcc-10
           - gcc-12
           - gcc-14
+          - clang-20
         include:
           - cc: gcc-10
             cxx: g++-10
           - cc: gcc-12
             cxx: g++-12
           - cc: gcc-14
             cxx: g++-14
+          - cc: clang-20
+            cxx: clang-20
     runs-on: ${{ matrix.image }}
     env:
       CC: ${{ matrix.cc }}