about: Explicitly state when app is not contained inside Flatpak

At the moment, whenever the user is not running under Flatpak,
the "Flatpak:" label won't show up, which confused me for a moment
when looking at https://gitlab.gnome.org/GNOME/gnome-calendar/-/work_items/1609,
as I assumed the "Flatpak:" label showed up whether it was running
under Flatpak or not, making me assume that they did not go in the
Troubleshooting section.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/777>

src/gui/gcal-application.c

@@ -152,8 +152,9 @@ build_system_information (void)
 
   g_string_append_printf (str, "GNOME Calendar (`%s`)\n\n", VERSION);
 
-  if (g_file_test ("/.flatpak-info", G_FILE_TEST_EXISTS))
-    g_string_append (str, "* Flatpak: yes\n");
+  g_string_append_printf (str,
+                          "* Flatpak: %s\n",
+                          g_file_test ("/.flatpak-info", G_FILE_TEST_EXISTS) ? "yes" : "no");
   g_string_append_printf (str,
                           "* GLib: %d.%d.%d (%d.%d.%d)\n",
                           glib_major_version,