downloader: expand user agent to contain OS release name

This should give us ability to debug problems with stucked versions.
8 jobs for expand-agent in 7 minutes and 45 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Test
passed #255038
cppcheck

00:00:45

passed #255039
luacheck

00:00:42

passed #255036
system-tests

00:02:05

passed #255037
system-valgrind-tests

00:04:04

passed #255034
unit-tests

00:01:23

passed #255093
unit-valgrind-tests

00:03:40

failed #255047
unit-valgrind-tests

00:01:11

failed #255035
unit-valgrind-tests

00:01:14