Follow-up for build pipeline

[benjaoming]

#99

1. Install build dependencies by using mk-build-deps: https://github.com/learningequality/kolibri-installer-debian/pull/99/files#r419932255

2. The test scripts should test the system service, not invoke kolibri start/stop.

3. Make artifacts public (403 errors when downloading directly with curl/wget)

CC: @DXCanas