Reliable way, in Rake, to determine whether we are being run by Fezzik

[cespare]

We should define a constant or env variable inside the fez command, so that it is easy to distinguish whether we are being run by Fezzik inside the Rakefile.

The main use case for this is to wrap the Fezzik loading stuff inside a guard which doesn't run if we're using Rake directly.

Right now the current workaround we've been using is

if ENV["fezzik_destination"]
  require "fezzik"
  require "path/to/fez/configuration.rake"
  ...
end

but this doesn't work in some case (for instance, when you run fez -T or anything else that avoids the code path that sets the fezzik_destination env variable.