fix: Set is_optimism to true if default config

[rakita]

If optimism_default_handler is enabled by default is_optimism flag will be true

[github-actions]

Valgrind Results:

==4018== Cachegrind, a cache and branch-prediction profiler
==4018== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==4018== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==4018== Command: target/release/snailtracer
==4018== 
--4018-- warning: L3 cache found, using its data for the LL simulation.
Snailtracer Host+Interpreter benchmark (2.1s) ...            not enough samples
==4018== 
==4018== I   refs:      471,787,909
==4018== I1  misses:          6,086
==4018== LLi misses:          2,908
==4018== I1  miss rate:        0.00%
==4018== LLi miss rate:        0.00%
==4018== 
==4018== D   refs:      229,165,034  (152,346,483 rd   + 76,818,551 wr)
==4018== D1  misses:        350,863  (    210,577 rd   +    140,286 wr)
==4018== LLd misses:        137,578  (      4,054 rd   +    133,524 wr)
==4018== D1  miss rate:         0.2% (        0.1%     +        0.2%  )
==4018== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==4018== 
==4018== LL refs:           356,949  (    216,663 rd   +    140,286 wr)
==4018== LL misses:         140,486  (      6,962 rd   +    133,524 wr)
==4018== LL miss rate:          0.0% (        0.0%     +        0.2%  )