test(turbopack): Use `mimalloc` for codspeed to make it realistic (#80877)
### What?
Use `mimalloc` for the codspeed benchmark to make it more realistic.
### Why?
I made a mistake while creating https://github.com/vercel/next.js/pull/79980. I'm retrying this to make
the benchmark more realistic.
Note: I didn't apply it to HMR benchmarks because it makes them more flaky.