Log the command line the collector is run with

WARN: don't merge before making tag 2.1.0 on backdrop-collector:
https://github.com/alphagov/backdrop-collector/tree/2.1.0

Note that this depends on a new feature introduced in
backdrop-collector-2.1.0

https://www.pivotaltracker.com/story/show/70748012

[#70748012]