system-tests: improve sorted compare

sort both aptly output and gold file. output original output for
debugging on failure.

* Makefile: enable CAPTURE=1 env variable for capturing gold files
* docker-system-test: use AWS env vars for S3 tests
* fix system tests timing issue with order of gpg logs in publish tests

system/t06_publish/PublishSnapshot38Test_gold

@@ -12,3 +12,4 @@ Now you can add following line to apt sources:
 Don't forget to add your GPG key to apt with apt-key.
 
 You can also use `aptly serve` to publish your repositories over HTTP quickly.
+