.gitlab-ci/buildstream.conf: use environment variables instead of relative paths

Part-of: <!1042>
11 jobs for abderrahim/buildstream-config in 72 minutes and 1 second (queued for 4 seconds)
Status Name Job ID Coverage
  Track
passed track #1170754
x86_64 gnome-build-meta

00:03:28

 
  Build
passed build-aarch64 #1171039
aarch64 gnome-build-meta

00:46:24

passed build-i386 #1170756
x86_64 gnome-build-meta

00:09:03

failed build-ppc64le #1170758
ppc64le gnome-build-meta allowed to fail

00:01:47

passed build-x86_64 #1170755
x86_64 gnome-build-meta

00:20:21

canceled build-aarch64 #1170757
aarch64 gnome-build-meta

01:50:30

 
  Deploy
canceled iso-installer-x86_64 #1170763
x86_64 gnome-build-meta allowed to fail manual

canceled pinebook-pro-image #1170760
aarch64 gnome-build-meta allowed to fail manual

canceled raspberrypi-4-image #1170762
aarch64 gnome-build-meta allowed to fail manual

canceled rock64-image #1170761
aarch64 gnome-build-meta allowed to fail manual

canceled vm-image-x86_64 #1170759
x86_64 gnome-build-meta allowed to fail manual

 
Name Stage Failure
failed
build-ppc64le Build
Fetching changes...
Reinitialized existing Git repository in /builds/GNOME/gnome-build-meta/.git/
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
Uploading artifacts for failed job
Uploading artifacts...
logs: found 2136 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=1170758 responseStatus=201 Created token=q8v7kaPf
ERROR: Job failed: exit code 1