allow using __optional with -fno-exceptions

[khangng-ampere]

Currently, including <exec/async_scope.hpp> will err:

include/stdexec/__detail/__optional.hpp:122:11: error: cannot use 'throw' with exceptions disabled
          throw __bad_optional_access();

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[ericniebler]

/ok to test 31e6bbb

[ericniebler]

thanks!