GHA: Linux: enforce the override keyword

[Al2Klimov]

https://stackoverflow.com/a/29879000

If every method declares if it overrides something, mistakes leading to actually nothing being overridden don't happen.

[Al2Klimov]

FAILED: lib/base/CMakeFiles/base.dir/application-environment.cpp.o 
/usr/local/Cellar/ccache/4.8.1/libexec/c++ -DBOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT -DBOOST_COROUTINES_NO_DEPRECATION_WARNING -DBOOST_FILESYSTEM_NO_DEPRECATED -D_GNU_SOURCE -I/Users/aklimov/NET/WS/boost_1_66_0 -I/usr/local/Cellar/openssl@3/3.1.1/include -I/Users/aklimov/NET/WS/icinga2/third-party/nlohmann_json -I/Users/aklimov/NET/WS/icinga2/third-party/utf8cpp/source -I/Users/aklimov/NET/WS/icinga2 -I/Users/aklimov/NET/WS/icinga2/lib -I/Users/aklimov/NET/WS/icinga2/build-clion -I/Users/aklimov/NET/WS/icinga2/build-clion/lib -I/Users/aklimov/NET/WS/icinga2/third-party/execvpe -I/Users/aklimov/NET/WS/icinga2/third-party/mmatch -I/Users/aklimov/NET/WS/icinga2/third-party/socketpair -Werror=suggest-override -Qunused-arguments -fcolor-diagnostics -fno-limit-debug-info -pthread -Winvalid-pch -g -DI2_DEBUG -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk -fcolor-diagnostics -std=c++17 -MD -MT lib/base/CMakeFiles/base.dir/application-environment.cpp.o -MF lib/base/CMakeFiles/base.dir/application-environment.cpp.o.d -o lib/base/CMakeFiles/base.dir/application-environment.cpp.o -c /Users/aklimov/NET/WS/icinga2/lib/base/application-environment.cpp
In file included from /Users/aklimov/NET/WS/icinga2/lib/base/application-environment.cpp:3:
In file included from /Users/aklimov/NET/WS/icinga2/lib/base/application.hpp:8:
In file included from /Users/aklimov/NET/WS/icinga2/build-clion/lib/base/application-ti.hpp:4:
In file included from /Users/aklimov/NET/WS/icinga2/lib/base/object.hpp:225:
In file included from /Users/aklimov/NET/WS/icinga2/lib/base/type.hpp:7:
In file included from /Users/aklimov/NET/WS/icinga2/lib/base/string.hpp:8:
In file included from /Users/aklimov/NET/WS/boost_1_66_0/boost/beast/core.hpp:15:
In file included from /Users/aklimov/NET/WS/boost_1_66_0/boost/beast/core/bind_handler.hpp:14:
In file included from /Users/aklimov/NET/WS/boost_1_66_0/boost/beast/core/type_traits.hpp:15:
In file included from /Users/aklimov/NET/WS/boost_1_66_0/boost/beast/core/detail/type_traits.hpp:13:
In file included from /Users/aklimov/NET/WS/boost_1_66_0/boost/beast/core/error.hpp:14:
/Users/aklimov/NET/WS/boost_1_66_0/boost/system/error_code.hpp:239:30: error: 'name' overrides a member function but is not marked 'override' [-Werror,-Wsuggest-override]
        virtual const char * name() const BOOST_NOEXCEPT
                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/system_error:213:25: note: overridden virtual function is here
    virtual const char* name() const _NOEXCEPT = 0;
                        ^

🤷‍♂️