Enable mimalloc by default in benchmarks

[Dandandan]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This keep it in line with datafusion-cli.

Describe the solution you'd like
Use the default feature. One could still disable or use sn-malloc instead by providing those as flags.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.