Backport: Turbopack: update mimalloc (#81993) (#82166)
We're heard about significant real-world reductions in memory usage from
this change in canary. It seem relatively low-risk, so IMO we should
backport it.
Original PR: https://github.com/vercel/next.js/pull/81993
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
