Disable Jemalloc for the debian package builds

Summary:
This cause the tests to segfault randomly on Launchpad. This has been
extensively tested on a supposedly identical Ubuntu box but was never
reproduced.

Test Plan: Ran the builds on my personal Launchpad and checked all succeeded.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7069