packaging: disable rust extensions again on CentOS
Backed out changeset eccbfa7e19c0
We're seeing (very rarely) crashes of 'hg purge' on some of our machines
(see https://bz.mercurial-scm.org/show_bug.cgi?id=6509 ).
Unfortunately, I haven't been able to find out much more about
what is going wrong.
To avoid further impact on our users and CI,
I would prefer to disable the rust extensions for now.
Differential Revision: https://phab.mercurial-scm.org/D10877
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