Allow customizing audit broadcast with env

When invoking the audit broadcast command, provide a few environment
variables so that people can customize the format of the message if they
want.

We currently provide `MRSK_PERFORMER`, `MRSK_ROLE`, `MRSK_DESTINATION` and
`MRSK_EVENT`.

Also adds the destination to the default message, which we continue to
send as the first argument as before.

lib/mrsk/cli/base.rb

@@ -73,7 +73,9 @@ module Mrsk::Cli
       end
 
       def audit_broadcast(line)
-        run_locally { execute *MRSK.auditor.broadcast(line), verbosity: :debug }
+        if broadcast = MRSK.auditor.broadcast(line)
+          system(MRSK.auditor.broadcast_environment(line), broadcast)
+        end
       end
 
       def with_lock