Defer calling the bl::format::str()'s c-str() until point of use.

Otherwise the string is a temporary that gets destructed at the end
of the statement and passing the char* to gnc_update_splash_screen
is a use-after-free.

Fixes macOS CI.

gnucash/gnucash.cpp

@@ -176,11 +176,11 @@ scm_run_gnucash (void *data, [[maybe_unused]] int argc, [[maybe_unused]] char **
         const auto checking = _("Checking Finance::Quote...");
         gnc_update_splash_screen (checking, GNC_SPLASH_PERCENTAGE_UNKNOWN);
         GncQuotes quotes;
-        auto found = (bl::format (std::string{_("Found Finance::Quote version {1}.")}) % quotes.version()).str().c_str();
+        auto found = (bl::format (std::string{_("Found Finance::Quote version {1}.")}) % quotes.version()).str();
         auto quote_sources = quotes.sources_as_glist();
         gnc_quote_source_set_fq_installed (quotes.version().c_str(), quote_sources);
         g_list_free (quote_sources);
-        gnc_update_splash_screen (found, GNC_SPLASH_PERCENTAGE_UNKNOWN);
+        gnc_update_splash_screen (found.c_str(), GNC_SPLASH_PERCENTAGE_UNKNOWN);
     }
     catch (const GncQuoteException& err)
     {