[ci] Use C++20 in march=native build

Like this we can test the SIMD features that are enabled for C++20 via
`std::experimental::simd`.

Author: guitargeek

.github/workflows/root-ci.yml

@@ -402,7 +402,7 @@ jobs:
           - image: alma9
             is_special: true
             property: march_native
-            overrides: ["CMAKE_BUILD_TYPE=RelWithDebInfo", "CMAKE_CXX_FLAGS=-march=native", "CMAKE_C_FLAGS=-march=native", "builtin_zlib=ON", "builtin_zstd=ON"]
+            overrides: ["CMAKE_BUILD_TYPE=RelWithDebInfo", "CMAKE_CXX_FLAGS=-march=native", "CMAKE_C_FLAGS=-march=native", "builtin_zlib=ON", "builtin_zstd=ON", "CMAKE_CXX_STANDARD=20"]
           - image: alma10
             is_special: true
             property: arm64