CI Linux: current live555 build fix

Version 2024-10-24 expects std::atomic_flag::test(), which is a C++20
feature but can be omitted if NO_STD_LIB is defined. (passing -std=c++2a
won't help because U20.04 libstdc++ doesn't contain that method).

.github/scripts/install-common-deps.sh

@@ -89,7 +89,7 @@ download_build_live555() {(
                 pacman -Rs --noconfirm binutils
         elif [ "$(uname -s)" = Linux ]; then
                 ./genMakefiles linux-with-shared-libraries
-                make -j "$(nproc)"
+                make -j "$(nproc)" CPLUSPLUS_COMPILER="c++ -DNO_STD_LIB"
         else
                 ./genMakefiles macosx-no-openssl
                 make -j "$(nproc)" CPLUSPLUS_COMPILER="c++ -std=c++11"