Can't rely on `lsb_release` for codename on pre-releases, like `buster`

`n/a` is not a helpful bit. This isn't necessary as jobs are release-specific, but it reduces confusion during packaging.
4 jobs for add/buster in 7 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test Pre Build
passed #2081
test_build_script

00:01:23

 
  Build
passed #2082
test_build_nginx_buster

00:04:18

passed #2084
test_build_nginx_jessie

00:06:32

passed #2083
test_build_nginx_stretch

00:04:37