pbs: enforce LC_ALL=C (importer/exporter as well) for proper sort collation

to mark the purpose, repeat inside the call to /usr/bin/sort,
but it's also needed for messages (i.e. we don't want /bin/cp
to write unicode chars in the pubsub feed)

This reverts commit 6909fe3b.

buildout can be run and may need libnetworkcache in order to run tests (if zc.buildout.testing.buildoutSetUp() is called).