build: Install demo agent to autostart directory too

We were doing this already but forgot to port this from autotools to meson.
3 jobs for wip/fix-agent-install in 14 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #24989
ubuntu-16.04

00:14:16

passed #24990
ubuntu-17.10

00:14:28

passed #24991
ubuntu-18.04

00:13:37