debian: skip using rust extensions for now since the debian rules aren't correct

Authored by spectral.

Description

debian: skip using rust extensions for now since the debian rules aren't correct

The debian rules seem to want indexers.so in
build/lib.linux-x86_64-2.7/hgext3rd/rust/indexers.so on my machine, but it seems
that it's not copied over properly, presumably from
build/temp.linux-x86_64-2.7/rust/libindexers.so. Since afaict Google is the only
user of these debian packaging rules at the moment, and we don't seem to need
this module, I'm just disabling it.

Differential Revision: https://phab.mercurial-scm.org/D3645