.gitlab-ci.yml: Do not pull source cache for vm build steps

It takes 35 minutes on intel builders. As a temporary solution until
this is fixed, we rmove the source cache from the VM tests.
Everything should be in artifact cache already so it should not need
it.
14 jobs for valentindavid/vm-no-source in 201 minutes and 9 seconds (queued for 15 minutes and 37 seconds)
Status Job ID Name Coverage
  Flatpak
passed #258508276
aarch64
app_aarch64

01:46:31

passed #258508277
armhf
app_arm

00:40:57

passed #258561765
cache_i686 x86_64
app_i686

00:55:36

canceled #258508278
ppc64le allowed to fail
app_powerpc64le

passed #258652742
cache_x86_64 x86_64
app_x86_64

00:51:04

failed #258557495
x86_64 cache_i686
app_i686

00:02:09

failed #258508275
cache_i686 x86_64
app_i686

00:00:09

failed #258625145
x86_64 cache_x86_64
app_x86_64

00:00:09

failed #258508273
cache_x86_64 x86_64
app_x86_64

00:00:09

 
  Vm
manual #258508283
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #258508284
armhf
minimal_systemd_vm_arm

00:02:37

passed #258508282
cache_i686 x86_64
minimal_systemd_vm_i686

00:01:21

manual #258508288
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #258508279
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:02:01