Packagers often want to use system jemalloc rather than the bundled one.

In the debian package for valkey 8.0.2, I found this patch that we could apply:

https://sources.debian.org/src/valkey/8.0.2+dfsg1-1/debian/patches/0004-Add-support-for-USE_SYSTEM_JEMALLOC-flag.patch/

@lamby Let's upstream it?