CI: disable Deckard under Valgrind

CI: disable Deckard under Valgrind

The test is useless in its current form because all the information goes
to logs, which are later discarded.

We need to fix issues detected by Valgrind first and then enable strict
mode which will not let the test pass if any problem is detected.

This might require recompiling LuaJIT in a way which is compatible with
Valgrind.