build: Correct python path

On debian-based systems, there is no '/bin/python3'.
3 jobs for wip/fix-autostart in 1 minute and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #25110
ubuntu-16.04

00:01:46

passed #25111
ubuntu-17.10

00:01:53

passed #25112
ubuntu-18.04

00:01:42