ci: Switch clazy job to debian-unstable image

Signed-off-by: Linus Jahn <lnj@kaidan.im>
10 jobs for master
Status Job ID Name Coverage
  Test
canceled #28770
allowed to fail
clazy

canceled #28771
qmllint

 
  Build
canceled #28775
archlinux

canceled #28774
debian-sid

canceled #28773
neon-bionic

canceled #28772
ubuntu-trusty

 
  Deploy
canceled #28778
allowed to fail
android

canceled #28776
linux-appimage

canceled #28779
allowed to fail
mac-osxcross

canceled #28777
allowed to fail
windows-mxe