Disable optional Jemalloc for the example Arch Linux build in build-unix.md

Summary:
Although jemalloc is available on Arch, this example is supposed to be
minimal and avoid using optional libraries. To have this example work out-of-the-box,
it should disable Jemalloc.

Test Plan:
In docker:

docker run -it archlinux:latest /bin/bash
pacman -S boost cmake clang git libevent ninja python
git clone https://github.com/Bitcoin-ABC/bitcoin-abc.git
cd bitcoin-abc
cmake -GNinja . -DBUILD_BITCOIN_WALLET=OFF -DBUILD_BITCOIN_QT=OFF -DENABLE_UPNP=OFF -DBUILD_BITCOIN_ZMQ=OFF -DUSE_JEMALLOC=OFF

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6933