Without setting JEMALLOC_OVERRIDE, we end up linking to two different
jemalloc builds. Once dynamically, as a transitive dependency through
rocksdb, and a second time to the static jemalloc that tikv-jemalloc-sys
builds.
This fixes dynamically-linked jemalloc builds, for the reasons described
in <https://github.com/girlbossceo/conduwuit/pull/400#issue-2316700200>.
This makes it possible to deploy Grapevine while using a database
originally created by Conduit, including leaving the admin bot user's
localpart the same as before.
This prevents us from needing to recompile the dependencies when that
environment variable changes, which generally changes on every commit,
which is far more frequently than the dependencies are actually changed.
Users of the nix package can now just use `.override` to choose what
features they want.
This also makes RocksDB automatically use jemalloc when Grapevine is
configured to use jemalloc.
