fix: Fix rare crash in signal handler

fix: Fix rare crash in signal handler

If the process is signaled after SignalHandler::~SignalHandler has run
then this assert will trigger:

    ccache: SignalHandler.cpp:87: static void SignalHandler::on_signal(int): failed assertion: g_the_signal_handler

Fix this by deregistering the signal handler function before destructing
the SignalHandler object.

Fixes #1246.